First time seton drain questions

Depends on how large the fistula is - think of it like a tunnel. So if it's a narrow tunnel, only diarrhea or a very small amount of stool should be able to come out. Otherwise it should come out normally.

It may be painful the first time, especially if you are taking opioid pain medication since that causes constipation. I would take a stool softener or some kind and drink a lot of water.

Keeping it clean and dry is the best bet. Actually, if stool comes through, don't worry about washing it out too thoroughly - that might cause more damage and introduce other bacterias since you aren't using sterile water. With the seton in, it should just keep draining and risk of re-infection is a bit low since the infection comes when stuff is trapped and there is no way for the body to expel it.

A stool softener is very different than a laxative. A laxative, which works within a few hours, will cause your intestines to contract and force out what's in them. You should probably avoid laxatives for now.

A stool softener works to prevent your intestines from absorbing as much water as they usually do, leaving your stools softer and a bit more watery, making it easier to come out.