A couple of my Neon's have laid eggs but never had any fertilized (they're in a community tank so get eaten pretty quick). I've read that Neon's are hard to breed but I suppose if the environment is right then it should be possible.
I would think a 20 gal would be big enough but I wouldn't have more than 40 in there, should be plenty of males/females. You would need lots of plants (preferably live) for them to lay their eggs on (mine used a java fern) and something like java moss for the fry to hide out in unless you can remove the eggs into another tank.
Not sure if Neon's actually pair up or if it's just a case of the female laying eggs and hoping for a passing male to come along and fertilize them, would need to read up on that.
I would give it a try if I were you, if it doesn't work out after a few months then you would at least have a nice planted tank you could move a couple of 'focus' fish in for more interest.