While seniority decides the exact who, Crutchley commanded because it was a joint force. It is quite normal that when two or more nationalities combine military forces, senior positions are allocated with an eye to the forces provided. The ABDA command is a good example of this early in the war. At Salvo, the Australians, while not able to contribute a large number of naval units, they provided their most important ones and were a very important ally in other areas. So it was 'smart business' to include them in the senior command structure under the US. Another compromise is if one nationality commands, that individual's deputy is from another nationality that is participating. It is more about politics and insuring harmonious cooperation than purely military requirements. Mixing forces of different nationalities under subordinate leaders reflects more military necessity than politics. Crutchley needed modern destroyers for his force and Australia didn't have any to give.My apology for the late reply. So I can see why Crutchley was CO of Aussie ships, but why was he in command of US ships? Was it because of seniority?
To look at ABDA, ADM Hart (US) was the senior naval commander. His deputy was RADM Palliser (RN). Command of the ABDA Naval Strike force was RADM Doorman (RNN). The selection of Doorman was controversial because many felt the Dutch lacked any modern naval combat experience. However, due to the political issues at play, he remained in command. Also note that Doorman not only had four US destroyers, he had a US heavy cruiser (Houston), a UK heavy cruiser (Exeter), two UK destroyers, an Aust light cruiser (Perth), and most of the Dutch navy under his command.
Pista! Jeff