I'll do a little math:
10 Guppies = 20 gallons
1 ADF = 1 Gallon
1 Betta = 1 Gallon
1 Snail = 2 gallons (technically 1 but they are messy)
In those last 5 gallons you can probably do 4 or 5 Cory Cats, 8 to 10 ghost shrimp (either cories or shrimp, not both. They'll compete for food), a few more ADFs (I find mine are happier in groups), 3 platies or sword tails.
There are some other options but those are more the ideal options. These estimates may be a bit on the conservative side but with stocking a tank Less is More. All suggestions are species specific, I'm not suggesting you add all of the above at the same time...
Last edited by Malvolti; 03-12-2011 at 07:35 PM.