Well, the short answer to your question is, no. The tank you have now is inordinately tiny for several of the fish you mentioned starting with the pleco and the Rainbow Shark
. Both of these guys are going to want 4' of swimming space, min. That'd be the 55g you're dreaming about.
The water stays brown because your bioload is so high, though the heavy filtration is doing you some good.
The tank is pretty much the minimum size for the gourami, but it should be fine if the unsuitable inhabitants are moved. The angelfish is also awkward. How old is this angel? Generally angelfish are shoaling fish and are happiest in groups of five or more unless they are in a mated pair. If it's an older fish I'd be weary of adding new angels since they are cichlids and can be mean to new introductions. If not, I might be tempted to add two more (this tank can't hold a whole shoal of five).
You've got some issues to resolve for sure. Can you give us an idea of your water parameters and decorations? Temp, pH, Hardness, live plants or not, etc.?