Tai chi is not a form of worship and cannot in any way be categorized as such. Therefore your problem with the Avodah Zarah aspect is simply solved.
However, there is a prohibition of
וּבְחֻקֹּתֵיהֶם לֹא תֵלֵכוּ
which includes all kinds of rituals the pagans practice. But this does not include rituals (chukim) that people do for a good reason, the rituals are only prohibited if they have no rhyme or reason (see Tos. Avodh zara 11b. see also Ran). In this case, even if tai chi is an old pagan ritual, you want the benefits of tai chi and therefore should be permitted.
The problem of Nichush should not apply here, since the one practicing it is deriving physical benefits from it and is permitted in such a case (even if some of the other benefits might be categorized as Nichush).