I have read a few posts where people were talking about using the new Fantastic 4 feat with LE Sue Storm and then use Thunderbolts to copy mystics and have the rest of the team be Wild Cards. Now correct me if I am wrong but you have to declare wild carded TA's at the time of attack right, so in this instance you would have to choose between the Alt Fantastic 4 OR Mystics right, you can't chose Fantastic 4 and then if the figure gets hit declare mystics can you?

This is correct. You choose one Ta per action, so it's either FF or Mystics.