I trained Fiona once and she turned out great. Her problem is that she's available only in a few chapters of the first part (only 2 or 3, i Think) and you should focus on training her in just these chapters (she also arrives at a very low level!); I think that it's pretty annoying, since she also has mov -2 in these chapters >_<
She has very nice growths and if you focus on training her she would be really nice. she was one of my top 5 units at the end! (Yep, I brought her in the Final chapters!)
I also use Edward EVERYTIME along with Mia (with data transfer); Right now I'm training Aran and he's turning out GREAT *_*, so you should definitely use him if you want someone that could tank for the others and only get 13 damage from Tiger Laguz units in 3-6 (if promoted, of course ^^)
For the others, you have to train Micaiah and Sothe anyway, so... =_= Oh, train Ilyana as well! I usually train Ilyana, Calill, Soren and Pelleas (awesome *_*) and bring them all in the final chapters: they're really useful!
User Haar if you want a unit that can do massive damage or Jill if you want a unit that can double. I usually train both and then bring only Jill (but right now I'm planning on bringing Haar... Because my Jill go screwed in defense °__°" well, even my Haar got quite screwed with Speed ._. I'm not so lucky ;www;) xD
Oh, you can use Oscar too. Get him to A support with Ike and watch them dodge everything they throw at them *_*