A 20long for its gallon capacity actually has a great foot print. I think that you can keep convicts in a community setting if you have the right community. I would however suggest a female convict if you choose to do that...they are typically less aggressive and heck theyre usually a bit more attractive anyway.
But I would actually lean more toward something a group of Dwarf Puffers, they typically dont exceede 1" in size but have every bit the personallity as a large puffer and they are totally FW. I could easily see a group of 8-10 in a 20Glong.