Avoid large/common plecos. If you want to go with a pleco, research their water parameters and get something that will coexist in similar waters as your other fish. In my 75g, which currently has 4 escondido (ranging from 3-6") 4 GT (with the largest at 6" and the others around 2-3") and 2 firemouths (just shy of 4.25") I also have 8 botia striata and 4 various plecos. The plecos are a king tiger, a clown, a spotted medusa, and a peppermint pleco. There are also 3 yellow peacock eels living in the tank.
Everyone gets along fine still, aside from some growing pains suffered in the escondido line. Once a pair in each species forms I'll be getting rid of the other 2 but it's taking longer than I anticipated. I'd definitely say the tank is a bit over stocked currently but heavy over filtration and weekly 66% water changes keep the water looking good.
You'll be fine with 1 gt 2 convicts and a smaller pleco as far as waste management is concerned...... as far as aggression is concerned, you should know by now it really depends on the personality of the fish. If any of mine were as bad as people on her make them out to be, I'd have dead fish all over the place. They are not as bad as people make them out to be and it's a really fun tank to watch.