Nice looking tank (: As I was reading I saw someone mention plecos, and I just though I'd warn you (in case you didn't know) plecos can grow to be HUGE, and most require a 55 gallon tank. Somehow, pet stores manage to push them off as the ideal cleaning fish, choosing to ignore their size.
I'd say go with cories or ghost shrimp. While ghost shrimp sometimes get eaten, they're relatively cheap (so start out with one or two before buying a boatload of them!). I've also heard that both cories and shrimp are very sensitive to water fluctuations, so the tank must be stable before adding them in. Good luck on your adventure! It will surely be just that, seeing as bettas are always giving you a run for your money :P Everyone has given you great advice about the tank, and a little bit of extra tweaking will make it a home worth getting jealous over.