Understandable. For X-23, I'm just sick of everyone being unimaginative and generic in choosing Summer Glau. I like her yes, but I find her a boring choice for the character, and I pretty much would just see River with Wolverine claws if she ever got the role.
With Hellion's hair, I like it short or long, I just would want it long enough to flow around his face when he's using his powers.
For Elixir, the only blonde pics I've found of him were the Twilight ones, and I didn't feel like people automatically betching because I had someone from Twilight in the cast. He really impressed me in S. Darko.
I never viewed the Cuckoo's as having no personality, hell Sophie was always more reserved than Phoebe, Celeste, Mindee, and Esme was the rebellious boy-crazy one. Plus in X-Men: The End mini-series(hate it if you want, I enjoyed it), it showed all 3 drastically different, so I'm sure even though their mainly sharing one mind so to speak, they'd each have their differences. And I think Brittany is a great candidate in that regard.
To be honest, the only one I'm not 100% happy with is Surge, but I can't really think of a good young asian actress that fits the role.