I like to wait 3 weeks before adding the male with the females after a neuter but that's me being paranoid.
As for the stitches, it depends on what your vet uses. If he uses internal stitches and glue for the outer skin, then I find my guys leave it alone but when my vet used stitches for the outer skin as well, then the boys would pick at it. But even if it opens up a bit, you just have to keep the little guy in a clean area and it will quickly close up. They heal sooo fast.