From what I have heard, generally speaking the Nazis (although some Nazis may have thought differently) regarded the Slavs collectively as racially inferior, but they seemed to have persecuted their enemies during WW2, namely the Poles and the Russians, much more than other Slavs like their allies such as the Romanians. Also, the Nazis didn't even consider some Slavs to be Slavs - they thought that Croats were of Germanic origin because they were allegedly descended from the Germanic tribe the Goths, Hitler considered the Czechs to be descended from the Mongoloids, etc. What is the most important thing to remember is that the Slavs who were considered to be fit to be Germanized weren't actually regarded as Slavs but people living in Eastern Europe who were descended from ancient Germanic tribes who still had their Nordic blood.
Hitler also opposed racial mixture with Slavs (except with those deemed racially on par with Germans). In April 1940 Hitler (through Bormann) instructed his Interior Ministry to dismiss any government official who had sexual relations with Poles or Czechs. After that time all Germans wanting to marry Czechs had to get permission from Nazi authorities. Ten days after invading Poland in September 1939 Hitler told Himmler that if any Polish POWs were caught having sexual relations with German women, the man would be shot, while the woman, while the woman would be publicly pilloried and sent to a concentration camp. In October 1940 Hitler again warned some of his closest associates about the perils of allowing racial mixture between Poles and Germans. In February 1942 Hitler issued a decree forbidding German soldiers from having sexual relations with Polish women. Any Polish woman caught having sexual relations with a German would be committed to a brothel.
When Germany began importing millions of Slavic slave laborers in the early 1940s, the Nazi regime did everything possible to prevent interracial sexual relations. In most cases they either sent Slavic women along with the men or else established brothels with Slavic women for them. They issued strict warnings to both Germans and the Slavic workers not to engage in sexual relations with each other. Starting in February 1940 all Polish laborers in Germany had to wear a symbol marking their pariah status. Every German farmer employing Slavic workers received a notice stating, "Keep German blood pure! That holds for men as well as for women! Just as it is the greatest shame to have sexual relations with a Jew, so every German who has intimate relations with a Polish man or women transgresses. German women who were caught having sexual relations with Slavic laborers were usually pilloried and then sent to a concentration camp. The Slavic male offender was executed.
Ironically, the Nazis did not automatically reject as racially inferior all progeny produced by German-Slav miscegenation. Hitler decreed in October 1943 that children of German men and native women in the Eastern occupied zones would be cared for by the German state, as long as they were deemed "racially valuable." These children were often taken from their mothers and sent to Germany to be raised by German parents or in German institutions. Likewise, if foreign women workers became pregnant while in Germany, Himmler directed that children deemed "good racially" would be raised in special homes in Germany, while those considered "bad racially" would be sent to separate institutions.
Richard Weikart, Hitler’s Ethic, The Nazi Pursuit of Evolutionary Progress
, pages 146-147.
Many scholars who have investigated Nazi policies toward Slavs have concluded, however, that they were driven more by economic necessities than by racial ideology, as though these factor were mutually contradictory. Sometimes these conclusions seem to derive from a lack of understanding of Nazi racial ideology, especially in relation to Slavs. Nazi racial theorists and policy makers did not—contrary to a popular misconception—believe that Germans were pure representatives of the Nordic or Aryan race. Nor did they think that all Slavs were non-Nordic. They claimed that both Germans and Slavs were mixed racial types. Though they considered most Slavs racially inferior to most Germans, some Nazi racial theorists claimed, surprisingly, that some Slavs were racially superior to some Germans!
When we examine Nazi ideology and racial policies toward Poles and Czechs, we find remarkable consistency between the views of German anthropologists and those of Nazi officials. Even before the Nazis came to power in 1933, German anthropology to a large degree had been permeated with similar racial ideals. Once the Nazi regime was established, anthropologists exerted considerable influence, both directly and indirectly, on Nazi racial policies. Gretchen Schafft remarks in her book, "The anthropologists' statements [during the Nazi period] and Hitler's program fit hand in glove."
When Hitler railed against Germanizing Poles and Czechs in Mein Kampf, he was criticizing a policy that determined one's membership in the German Volk by linguistic or cultural criteria. Otherwise he stressed the preponderance of race. Hitler never discussed in sufficient detail his views on the composition of the Slavic races and his position on Germanization to determine if the Germanization policies after 1939 marked a departure from previous ideology.
This is probably the clearest statement by Hitler before 1939 concerning his racial policies toward Poles. However vague is Hitler's statement, if we compare it with actual Nazi policies toward Poles after 1939, we will observe continuity. As regards isolation, the Nazis did their utmost to prevent miscegenation between Germans and Poles. They deported hundreds of thousands of Poles to make space for ethnic Germans resettled from the Baltic States, Bessarabia, and Bukovina.
However, the differences in treatment of the Czechs and the Poles may have been driven neither by economic considerations nor by racial ideology. In a pamphlet published by the Racial Policy Office and meant exclusively for Nazi Party officials, Egon Leuschner discussed the ideological underpinnings of National Socialist Policy toward Foreign Peeoples. Leuschner claimed that his pamphlet represented the Nazi Party's official position, and the preface was written by Walter Gross, the head of the Racial Policy Office. Leuschner denied that Czechs and Poles were being treated differently based on their racial composition. While he acknowledged that a higher percentage of Czechs than Poles could be Germanized, he did not claim that the differences were due to economic considerations. Rather, he asserted that it was because of contingent historical events, especially the way the two countries were subdued by Germany.
First, the Nazis deemed the vast majority of Poles racially inferior. Second, according to Nazi racial thought, Poles with Nordic racial features, if they refused to abandon their Polish identity, were actually more dangerous than those of the inferior East Baltic race. Destroying the Nordic leadership of the Poles was thus essential to keeping Poland under control. Nonetheless, as I have shown above, the Nazis did hope to Germanize as many Nordic Poles (and other Slavs) as possible, as long as they would cooperate. Leuschner confirms my interpretation of Nazi policy toward the Slavs by rejecting the view that Nazi policy toward Slavs was on the whole haphazard or inconsistent. The whole point of his pamphlet was the exact opposite: to show how Nazi policy was consistent with its racial ideology. He explained that Germans, Poles, and Czechs contained a mixture of races. While the Nordic race predominated among the Germans, the Poles and Czechs belonged mostly to the Eastern and East Baltic races. However, the Poles and Czechs also had some Nordic blood, especially from German migrants in the past who had adopted the Polish or Czech languages. These Nordic Slavs could be reincorporated in the German Volk, but the long-term goal for the bulk of the Slavic population was deportation from lands conquered by Germany. Leuschner admitted that wartime economic necessities made this goal unattainable for the time being. He further argued that even after the war it would take a long time to carry out the said policies. Despite this intervening delay, however, Polish workers in Germany during the war were identified with an insignia to keep them from mixing with Germans.
The historians who have argued that the Nazi regime set aside its racial policy in formulating policies toward Slavs evince a slight misunderstanding of Nazi racial ideology. Nazis did not consider Germans or Slavs pure racial types, but mixtures of several European races.
Anton Weiss Wendt, Eradicating Differences: The Treatment of Minorities in Nazi-Dominated Europe
, pages 62-78.
John Connelly's short pamphlet Nazis and Slavs: From Racial Theory to Racist Practice
is an excellent source for someone to read about how the Nazis treated different Slavic ethnic groups. Slovaks and Croats were allowed their own puppet states. Although the Czechs were treated worse than the Slovaks, ordinary life for most Czechs was 'normal' - despite Hitler's anti-Czech views which were formed during his early years in Vienna. Poles were treated awfully right from the start of the invasion of Poland which started WW2. The Nazis did not occupy Bulgaria by military force and even allowed the Bulgarian government to do their own thing with regard to Bulgarian Jews. Slavs who lived in the Soviet Union were subjected to a policy of extermination as soon as the Germans had invaded the Soviet Union in June 1941 - this is striking because many Ukrainians initially regarded the German soldiers as liberators, although this was very short-lived due to the actions of the German soldiers.