[@Gowi]

As far as I can see theres no problem mentioning the group if they have not explicitly been excluded (which they have not).

But dont write them into your arc, if they do not currently exist (which they dont)
If you need them for your arc to make sense - apply for the character or find someone else that can commit to it. 

Just mentioning them for the sake of filling out a post, when no Xmen characters are being played, doesnt seem to add up. If they are more than just a passing mention - you need those characters to actually properly exist in this World (by your hand or someone elses) before including them.

[@Gowi] - Im not saying you're mentioning was wrong - this isn't my game, Im just saying. In line with what I can see Morden has said - Just makes sense that the characters exist properly - via application before their inclusion in an arc.