Can I add 2 killifish to this 3ft 40gal.?

You have a couple problems. Bumblebee gobies are brackish, and they'll slowly die in freshwater. Killifish are tiny, and will be a snack for the angels and possibly the gouramis. Add more neon tetras. They like to be in schools of 6 or more. Same with the black neon. However, I'm willing to bet that if you add 3 black neon tetras, they'll school with the regular ones.