I have

1 bristle nose pleco
2 Cory cats
3 Australian rainbow fish
4 dwarf gourami fish
5 pristella tetra
6 mollies

Are these guys all comparable with each other and what problems do I have with these?

Thinking of adding another school of something, any ideas?

I personally advise against more than 1 dwarf gourami, probably because mine has a nasty temper and killed my other one. Get more corys, I have 3 and i plan to buy more. Maybe a few more pristellas too, I have those as well

1 bristle nose pleco- OK as a single, male can claim a cave for breeding. 1m+1f are OK, but not 2 males.
2 Cory cats- Social fish, half a dozen or more is good.
3 Australian rainbow fish- Boisterous fish. 1m + 2f, or more, in the same ratio.
4 dwarf gourami fish- As noted above. One. Most Gouramis are aggressive and territorial about their space.
5 pristella tetra- Schooling. A dozen to 2 dozen would be good.
6 mollies- Hard water fish, not really compatible with the rest of your list.

My experience as well. I once had a community tank with a pair of dwarf gouramis, a pair of pearl gouramis, and other fish. The dwarfs began breeding and the male killed both pearls. After the eggs were laid, he went after the female dwarf.

That is typical behavior for Gouramis. They see the females as potential egg/fry predators. They see all other species of Gourami as competitors for their territory.
Some Gouramis are more aggressive than others. In a 55 you could stock 1 of the more aggressive types- 3 spot (or color morphs), Dwarf, Betta splendens, Paradise Fish. Some of these are so aggressive they will attack other species (non Anabantoids) that swim lower in the tank.
Moonlights, Pearls and Honeys are less aggressive (mostly) and you might try 1M + 1F of Moonlights or Pearls, or even 1M + 2-3F Honey Gouramis. Have lots of hiding places.

The other thing to consider about stocking fish is their requirements for temperature, water parameters, water flow and so on.