1) They probably won't be using Tara Strong because she was already in an FF.

2) If they did, she'd probably be Penelo.

3) Tara Strong is actually a very good voice actress, and does many roles on American TV, such as Raven from Teen Titans and Timmy on the Fairly Oddparnets and many other things. I wouldn't call her a bad voice actress just because you disliked the way she did Rikku, as she does many other roles very well, I'm not making a full list though. XD

4) And, when they have english voice actors, i'm sure you see a thread where someone will list them for you.