Unless you just like Otos, I would go with 2 cory cats in a new tank because you won't have algae anyways. The cory cats will help keep your gravel clean
You probably won't need more than 2 otos to clean the algae in a 20 gallon. If it's a brand new tank with no algae you need to supplement the otos with algae wafers or fresh Zuccinni slices...they'll starve otherwise
you're right, the more the better...but as long as they aren't alone they seem ok. I had a pair once and they seemed like happy little campers and best friends.