What filter do you have on this tank?
What kind of gourami?
What kind of pleco?
I think two things are happening here:
You are probably underfiltered for the amount of fish you have and the bacteria don't have enough places to grow on and mostly you're way overstocked even for a cycled tank. The Severum and Chocolate Cichlid can grow very large (10+) inches and are going to outgrow the tank; in the meantime they're putting out a lot of waste. If the pleco is a common, it can grow up to 18" and they are very messy fish in general.
The only things you might be able to move to the 30 would be the gouramis if they are dwarfs but it woudn't make much difference in the toxin levels with the other large messy fish you have and then you'd be doing water changes on two tanks.
First let us know your filtration; adding more might (MIGHT) help but in the long run you're going to have to either upgrade to a massive tank or start rehoming some fish. Sorry.