Definitely don't get otos! They are really sensitive and generally don't do well in less than 20 gallons. There are exceptions, as always, but they aren't the rule.
If you want cories, I would go with 4 pandas or 6 pygmy cories.
If your betta is chilled, you could go with tetras. I strongly recommend embers, as they are tiny, so the betta won't see them as a threat and they have a smaller bioload, they aren't even slightly nippy, and they are very peaceful, so the betta won't be stressed out by any frantic tankmate activity.
If you are super good at water quality maintenance, 6 pygmy cories and 6 embers with a betta would stock you. That wouldn't be overcrowding on any level, at least. :)