I've got two ghosts in my DPs tank and so far, she ignores them. Maybe because they are bigger than she is?
I feed her f/t bloodworms almost every day, and drop a snail or three in every week. She's not as fond of snails (or hasn't figured out they are edible) cause I still see them around. The ghosts will come running to snag a bloodworm, and I've got other sinking foods I'll toss in a few days a week. Maybe keeping everyone well fed is the trick? Or she's just really mellow?
I have had GS in with my 3-5 dwarf/pea puffers (DP) for years. The ghost shrimp (GS) in there right now are at least a year old. The DPs are also fed bloodworms daily and crushed ramshorns once or twice a week with a skipped feeding every week or so.
If you want a good cleaner get a couple nerite (Nerita natalensis) snails. Their foot is small and the puffs can't bite it. I have 4 of them in my F8 puffer tank.