They're not a big waste fish...they're not real territorial or aggressive...I'd say you could put as many in there as you feel like up to (or a little over) that 1 inch of fish/gallon rule. I had 5 in a 2 gallon for the longest time and they did great.
Maybe mix in some different kinds and see if they school together...one of my lps has some gorgeous long finned and leopard varieties that I'm thinkin about getting. I also hear that they're pretty easy to breed if you have the right circumstances for them...so maybe stick with 20 or so and see if you can get them to breed??
And yes, I would agree with leopardess. They seem like the type that would love a current.