Oh gosh, I would have a hard time letting him go too.

Just my opinion but I think your daughter sounds very responsible and is at a good age to take this on. As for Lexie, I think the key would be to introduce them slowly - if you haven't already - and to keep reassuring both Lexie and Ash of their positions as top kitties. Be sure to give them lots of extra attention.

Please let us know how it goes.