There were FAA licenses pilots in Tiger, although they left. Some went to JetStar. I'm not sure, but I think you have to do the same as a JAA license holder: Singaporian air law examn, and - if type rated already - the A320 technical test. If not type rated, you will do the conversion within Tiger.
I didn't hear of any problems for Americans. There were also Mexicans and other Americans in Tiger.
hth,
Dani