Thanks for the help and link, everyone.
@shellieca- I don't think I'd consider my otos bottom dwellers. They seem to prefer mid level, on leaves and the glass. But you're right, there is quite a varied opinion on size requirements for kuhlis- I've seen everything from 10 gallons is okay, to 30 gallons minimum. I suppose it's a bit like the debate on tank sizes for bettas, eh?
@Bombalurina- I think you're right and it would be better for the fish if they had larger groups of their own kind. Rather than try and squeeze as many different types in as I can. After all, they're the ones who have to live there.
So, it looks like I'll be sucking it up. :) Or...
Plan B- get my husband liquored up and, then, sweetly ask him if we can get a 20 gallon....