A 5 gal is too small for a betta and other fish.. as the bio load would be too much, as well as some bettas get territorial in a tank that size.
I could recommend a few other things you may add since they won't be taking up the same space as the betta..
Ghost shrimp, cherry shrimp, etc would be good in there, only a few.. their bio load is rather small so it won't impact the tank as much. Just be aware that some bettas like to snack on them.
You could also get one snail, but be aware you may have to do a little more water changes per week as they are dirty little guys.. and only get one, otherwise within a few weeks you will be over run.
You could also do 1 African Dwarf Frog as they are small and their bio load is also pretty light. They do better in small tanks such as a 5 gallon because they really aren't all that good swimmers. But it may be harder to feed them and not over feed your betta. Frozen or Freeze Dried Brineshrimp, Frozen or Freeze Dried Bloodworms or Glassworms, HBH Frog and Tadpole Bites, Reptomin, Gammarus, Frozen Beefheart, Frozen or Freeze dried krill or baby shrmp, Earthworms or nightcrawlers choped into small bite sized pieces are what you would be feeding them.
They also need places to hide as they are shy.
Pretty much that's it. A 5 gal just won't hold too much unfortunately. Maybe others will have more ideas to help you out =)
Or.. you can make a divider and have 2 bettas in there if you desire. DIY divider