As @nikv will confirm. It takes a few months for the hormones to subside after the operation and hopefully the aggression with it.

As far as litter boxes. Just place more. It has worked for us. Multi cat household here with 8 of the little fur monsters. We got fancy schmansy cabinet ones. A couple igloos. The more boxes the better. Pain in the ass but solves the outside the box issues. Our latest rescue pees on his back paws then tracks it. Still trying to fix this issue. He uses the box. But is weird about it. He has gotten better at it tho...