Neutering them won't necessarily stop the humping. It is actually showing dominance, it's not a sexual thing. Missy humps her favourite stuffed animal and she's spayed
I've had a few spayed female dogs that have done this. A good reason to neuter your boys when the vet advises is to decrease the chances of them marking in your home.
Congrats on your new little ones. Hopefully the boy with luxating patella doesn't require surgery.