Sorry for the long wait for the answer. RL kept me beyond busy.
1) As long as both feature in the story you write, you can claim what you want. Meaning if you want to claim a crossover pairing all fics should feature that pairing - beyond that there's no restriction when it comes to claiming. -- Long answer short: you can claim to fandoms for one claim.
2) As long as the post isn't friends-only you can do a fake cut.