I have two otos that seem to be doing just fine on their own. I actually don't feed them very often, and they're still happy and fat (otos should always look a bit round in the stomach).
When I put veggies in, I parboil them (bring water to boiling, shut the heat off, then drop the veggies in for a little bit), freeze them, or just stick them in the tank and leave them for awhile. I think that what most fish like is when the cell walls on the plant start breaking down, and freezing or parboiling will begin this process, or it will just happen over time as the veggies sit in the tank (usually 1-2 days). Of course, the last option releases extra waste into the tank, but it's the easiest of the three

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Microwaving for a few seconds probably works just as well.
I've seen pictures of cories in a betta tank, and they seem to do fine. The cories are peaceful, and are unlikely to bother the betta while he sleeps, and their too big for the betta to think of as food, and the wrong shape to be an enemy. The betta may try to chase the otos down (Keiji did), but the otos are likely going to be far too fast for him to get them. As long as they have some place that they can wedge themselves into, between, or under, that the betta can't go, they should be fine.