Clown and bristlenose plecos average about 7 inches full grown, I can say this first hand from 10+ yrs of experience in raising and breeding them. They also are known for the amount of food they require, they have huge appetites.
As for the firemouth and convicts.. if you are keeping 1 of these in a tank, yes, you can get away with 30 - 40 gallons for each of them, but together, not. The other problem is going to be aggression levels as the fish grow. The convict cichlds are extremely aggressive and territorial, and are likely to kill any of the others you've listed if kept together long term in a 40 gallon tank. My last female convict had to have a tank to herself because she ate everything, including a 7 inch oscar... she was only 3 inches at the time, and this happened overnight after 3 months of peace. She was there temporary, waiting for someone's tank to be ready for her... nothing survived that fish and that was in 75 gallons.
As for the parrot fish, active or not, they get large and need space, territory, and clean water. With all of those fish in a 40 gallon tank long term, all that will happen is they will slowly poison themselves with the waste levels that are unavoidable, and tear each other up as they grow and need more space.
As for discus, they need a very large tank (minimum of about 75 - 90 gallons just for a pair). Discus average about 8 inches around when full grown. Discus also need very peaceful tankmates. None of your fish qualifies for this except the plecos. Discus also require a pH of about 6.0 and can't survive in harder water. Discus are not an easy fish to keep, but there is no way to keep them mixed with what you have in any size tank. Discus by themselves should start as small in at least 55 gallons for 2 - 3 of them and move up from there.
I know this isn't what you want to hear, but there is no way to make that work. That is not possible no matter how you attempt it. Instead of getting more fish I would suggest working on properly providing for what you already have.