Let me explain my thinking:
There are two factors involved in stocking, the first is waste load. You add more fish, you have to feed them more, you're adding more to the system which ends up as nitrate, phosphate, etc. which need to be removed by water changes, plants.
Being that I go buy 25 gallons a week of RO water for my 75 gallon tank, I wouldn't have trouble doing sufficient weekly water changes to keep the nitrates down.
It's not such a problem with nitrates as it is with the ammonia and nitrite, which are toxic to fish from .25 parts per million and up. In such a small tank the parameters are less stable and there is a greater chance of a toxic ammonia or nitrite spike which will quickly kill off otos as they are sensitive to water quality, and your betta's fins will be more likely to rot and get infected.
The second is crowding, and this is mainly why I'm asking here what I should add. Obviously active, mid-level fish will crowd out other active, mid level fish more than a loricariid or shrimp would crowd out a surface dweller. I think things that stay fairly small (2" and less) would do fine in this respect. This is why I said that ottos are the obvious thing I could add, however I am looking for more options, as always.
This is true, but you need to take into account the aggressiveness of the betta and the bioload of all the fish you are adding, which takes me right back to the fact that smaller tanks are less stable parameter-wise and if you put in more bioload than the tank can handle you will end up killing EVERYTHING or atleast damaging it in some way. The only thing you COULD add would be some shrimp which may end up as food for your betta.
Of course, I'll set up the tank with only the betta and some filter squeezings from an established tank before adding anything else.