Hand tight plus 1/8 to 1/4 max, the trick is everything clean and you should have no problems. If you overtighten they may not snap today, but they for sure will snap when you are on vacation
There are different qualities and schedules of bulkheads, but sometimes we are stuck with a certain hole size. Most are cheaply made out of ABS, but you can still find quality schedule 40 PVC bulkheads. If you are running closed loop higher quality are recommended and on all my closed loop plumbing I do run a bead of silicone around the outside of the flange after the bulkhead is in. I do this on the wet side only. I also like slip fit on the dry side with glue, trying to tighten threads to get a leak to stop can spin your bulkheads and unset the seal causing a leak. I try to stay away from threads whenever I can as they are where you will find most leaks. I have a couple sources for PVC bulkheads at work, hit me up if you are having trouble finding them for your closed loop.