Just lightly snug is fine. Its not for holding water by the looks of picture. No need to be waterproofed. Don't want to risk cracking the bulkhead. Those pvc fittings are designed to fit into that brands other fittings. It probably won't go all the way.
If I were you I would add thread sealer to that if you haven’t already. Actually if that is just your drain to the sump and there is no pressure you should be fine just the way it is.
As already stated, you don't need to run the threads all the way to the bulkhead. This has the possibility of cracking the bulkhead. Hand tighten and then about 1/4 to 1/2 turn. Also suggest using a pipe thread compound (made for PVC) on the threads. Don't use Teflon tape.