I always like to think of levels when deciding how many of a particular species to put into a tank. For instance, Otos are a fish that live most of the time on the bottom of the tank. You already have 8 cories that will be occupying the same area. So you have to decide how many more bottom dwellers could you put into a tank that size. I think that depending on what kind of cories they are you might be okay with putting 3 or 4 Otos (3 I feel is the minimum for that fish).
I wouldn't do them with angels. I have heard of angels attempting to swallow them and getting them stuck in their mouths, resulting in the death of the oto and the angel if the oto cannot be removed from it's mouth.