a group of most type of corys would work in a 5gal as long as the filtration is ok. Just remember to wait until the tank is fully mature before you go filling it with anything else. Leave it with the betta in for at least 2 weeks.
Some tankmates that could go with bettas..
- Corys
- Otto Catfish
- ADF (NOT ACF!)
- Gobys
- Dwarf Chain Loaches (if you can find them!)
There are more but these are the most commonly available ones. You would be surprised what works well with bettas, I have kept them with many things but they simply don't interest me much anymore.
Ben