Hey dude,
I admire your taking the fish in, even though you didn't know what they were exactly. Good for you for trying to do the right thing.
I would go ahead and take them to the lfs if they seem to be concerned about the size tank they really need. If they don't, I'd try to phone any public aquariums in your area. If that doesn't pan out, keep them a while and try to find a better home. If you haven't by the time they've outgrown your tank, take them to the lfs if they'll take them, or euthanize them if they won't. By then they'll be too big for just anyone to go in the lfs and decide to take them home on a whim. .. buying a 5-6" fish tends to make people think on it a while before purchasing it, kwim?
Anyway, good luck with it... it's going to be a stressful I'm sure. Don't let it bother you if anyone tried to make you feel stupid for getting them. You did what you thought was best.