A twenty gal long tank in my view, would not be suitable for a pair of convicts UNLESS you have plans for the many babies they are capable of producing. The tank would soon be overstocked and no doubt underfiltered.
There are many pleco species. Some grow no longer than five or six inches while others can and DO reach eighteen inches or larger.
"I know a guy,, blahh,blahh, blahh. Most responsible fish keepers have only the best interest of their fish in mind when selecting which fish they will place in their community or species only tank And would not be interested in those whose irresponsibility and usually ignorance, posesses them to keep fish in unsuitable enviornments.
A twenty gal long would be suitable for german rams assuming the water conditions are in their favor. they are not the easiest fish to care for in my opinion.
The Bolivian Ram
is a bit hardier and would be suitable for twenty gal long.I just gave a twenty gal long to neighbor and never liked the lack of depth of that particular tank. I kept Kuhlii loaches in it ,and when I moved the kuhlii's to larger quarters ,I placed corys in the tank along with some fancy guppies.
Larger fish will ALWAYS do better in larger tanks.That's my story and I'm stickin to it.