If you do end up getting more girls and a bigger tank, which would be what I'd recommend (just be careful introducing them and quarantining) you might be able to get a neat snail or something. :) I'd suggest otos but they're not quite beginner fish, they tend to die off easily when you first introduce them and I don't want to put you off fish owning. In a sorority, I think ghost shrimp would get eaten... and don't get a chinese algae eater, they are aggressive and get really big. Plenty of plants and caves and you're ready to go.
Check out the eBook Betta Fish Care Made Easy
