20 Gallon Brackish Stocking

[Lunatic430]

Hi, I’m looking for fish to stock my 20 Gallon tall tank. The first fish I had in it was my GSP, I had seen that you could put other fish in with a puffer, but I had also seen people keep them in community tanks. So I bought two black mollies and three guppies, once they where acclimated, I put them in the tank, and my GSP could care less.
So, does anyone have recommendations?
Thanks!