Adding more fish, what would you recommend?

No, female bettas don't get along in small groups. They create a hierarchy and the one lowest on the totem pole gets picked on. In bigger groups 3+ the punishment gets spread out, but in pairs, the lower one will get picked on to death. You'd need 3+, more likely four and a lot of hiding places.

Plus its questionable how well the gouramis and the bettas will get along.

I agree, your tank sound stocked to me.
 
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