How do decide which costume?

[JOker]

Costumes are split according to their allegiance, basically, those allied to the light side (rebels, republic troops, etc) fall into the Rebel Legion, and those allied to the Dark side (Sith, Stormtroopers, bounty hunters etc.) fall into the 501st.

Some characters are "dual approvable" meaning that their allegiance is undetermined (such as tusken raiders, jawas, wampas and the like) or they have clearly served both sides (Clonetroopers, thanks to the whole order 66 thing)

You can check out the full details on how the membership team determine what is acceptable into the 501st at this link:

http://www.501st.com/databank/Costuming:...erApproval