If you feel you have enough algae or are able to provide a supplemented diet for more than one otocinclus, I think you should bump your numbers up to 3-4 individuals.
Otocinclus do seem to prefer being in each other's company as they are a shoaling fish by nature. I have found their bioload to be fairly negligible in a cycled tank. However, the difficulty is in ensuring that they are all well-fed as it is not uncommon for them to refuse to transition over to other food.
Depending on your betta's propensity towards aggression and the capabilities of your filter, you may be able to get away with a school of small tetras or rasboras. Just remember that there are more species out there than just neons and some are very suitable tankmates for a particularly gentle or indifferent betta.
Downside is some of these species can be sensitive so if water conditions aren't to their liking they can rapidly decline in health and cause all sorts of problems.
Do you know what the hardness/softness of your water is along with a pH value? That can help with selecting appropriate tank mates.