[QUOTE]the tai chi around me is mainly for health, will it still be useful?[/QUOTE]
If they teach the principles correctly, yes.
If I am right, I thought that BK Frantzis learned Xingyi when he was in Tapei, Taiwan. Sure some of the other students had Karate 'dans'- however, the Karate taught over there is much different than the majority of the Karate that you see in the states.
Personally, I wouldn't recommend an EMA before going into an IMA, however, if an EMA is all that is available, then you will just have to make due with what you have.
The reason I would learn Taiji is because you don't have to unlearn some of the habbits that you will inevitably pick up with EMA training. Habbits that will impeede progress within Xingyi.