I like Terra and she does bring star power but she's another blonde. That's why i said add Traci (brunette) or Anita (red head) to contrast Cassie.
My only concern with it is that the YJ generation don't have the star power that the original and New Teen Titans generation does, so Tara adds a bit of that. I'd probably have their hairstyles be different, too - give Tara a bob and Cassie long hair.
"We come into this world alone and we leave the same way. The time we spent in between - time spent alive, sharing, learning together... is all that makes life worth living." - Jean Grey