Congrats on the Common, they're pretty cool!
Have you tried to feed him veggies yet? They're a blast to watch eat zucchini!
I used to keep mine in a 20 gallon tank, which lasted about four months before I set up my 40 gallon. Now he lives in there and it's been a good eight months. He's grown maybe 2 inches since I've gotten him and I overfeed him because I can't resist watching him inhale zucchini. I personally think you'll be okay with the 20 gallon for a little while, but only if you're filtrating it well, have places for him to hide and stake out a territory in, and as long as you're aware that he will need a bigger tank when he hits about 5-6 inches. I bet he'd like a pond when he's bigger :3 Good luck with him!
And now, a little rant because it's midnight and I feel like putting this out there: I dunno, I just get a little... not annoyed, but bothered when people jump on others when they have commons in smaller tanks (not that anyone in this post did, that's a very general statement and when I say smaller, I mean 20g, not 10, that
is too small). True, they need to realize that the Plec is going to need larger housing as it grows (which they grow pretty quick), but sometimes, people make it sound like the owner is going to wake up the next day and the Plec is going to be 2 feet long and sucking on the walls or something. Hmmm... sorry, I'm just in a bad mood and that's always bugged me a bit. Just putting it out there... that wasn't directed at anyone in particular. I'll stop my rant now, sorry, don't eat me D: