Yes, The 6" I saw where old ( how old idk but at least 5+yrs..) and they where in a well planted 180 set-up. Same guy had some 10"+ clown loaches and i'm assuming he fed them a lot ( I know lots of blackworms where fed to this tank) But I would agree that in the average aquarium they would reach 4"+ easy ( I've had them come in from the distributors at work already about that size)
imo the main thing to be careful of is people do not realize how easy it is to overstock a 20-55gallon tank. and even some fairly common fish ( such as the glass cats, thought oddball they are) can often need the larger end of the spectrum tanks.
Be wary if you go w/ sailfin mollys... yes they breed like mice.. I had one female at one point that would frequently drop batches of 200+fry in my 55.. I'de get home and suddenly there was a "cloud" of fish in the tank... But they are semi-aggressive fish threw and threw. The males can be downright nasty killing anything else in the tank they can get ahold of ( not as bad as cichlids but any peaceful fish like glass cats ect would get picked at until they died from stress very likely) I also had a big 6" male.. stunning... but again I wouldn't imagine trying to keep him happy in a 20H... if you went w/ semi-brackish black mollys would do well in a 20h imo, golddust also stay on the smaller end... where the lyretails can get pretty good sized. I would honestly pick platies over mollys for a 20... they stay abit smaller and come in a wide array of colors as well ( imo even more)