Personally, I'd leave things at the stocking level they are now. Small volume tanks can rapidly change their water quality and chemistry, far harder to carefully maintain than say a 240l with reasonable stocking.
Bettas don't like to be crowded, and on the other hand tetras can nip at them. I would add a few more cardinals if you really want more fish, but you don't have room for a whole new decent size school of fish.
In terms of space, you could certainly up your numbers a wee bit, but since your filter is only rated up to 60l, I wouldn't go as far as Zod said originally.