My community tank has a mixture of gouramis, (not advisable really with a betta but it is working for me. I have a balloon kissing gourami and a red gourami male and female.) I have a dalmation molly, a lyretail molly, several guppies, upside down catfish, 4 corys, 2 common plecs, 4 otos, 2xdwarf puffers, lemon tetra, 2 bleeding heart tetras, 2 pakistani loaches and Sinatra my male Betta.
I would say in a 10g the fish that would be compatable would be ones that stay small such as mollys (there are some nice ballon mollys.), swordtails, (guppies may not be a good idea in such a small tank as the Betta may see their tails as a food source), dwarf puffers, platties, some kind of tetra (there are loads of breeds of tetra).
I would also advise that you get the 10gallon tank, cycle it for 10 days or so with everything in (plants, gravel and filter) then add your Betta to finish off the cycle (this is what I did with my tank.) after another week, providing your parameters are ok, you can add another fish. Don't get too many though, a reputable lfs should be able to give you an idea of how many fish you can put in this tank and they should be able to tell you how big the fish you are looking at should get. This may sound silly but I would take a small ruler out to the lfs with you so that when they start saying how big the fish will get you can see exactly how big it will get to so you don't have to guess.
Kaz
p.s: I am sure there will be others on here that have a different view as to what will be compatable. It seems that what works for one person won't always work for another. Like i said I have Gouramis in with my betta and have had no problems.