I am afraid one oto would starve in a 5 gallon tank let alone two. I can really sympathize with how badly you want them. 5 gallons is just so small, very easily fouled by fish waste and leftover foods, and can only be filtered by a very small filter without blowing the fish and water out of the tank.
The 1" of fish to 1 gallon of tank water is bunk. It certainly depends on the fish. I don't keep guppies or shrimp so I'm not sure what kind of a bioload they have, in my opinion just 4-5 guppies would be stocked full in there. Hoping people more familiar with guppies and shrimp can lend an opinion here too.
How is your snail building algae levels? Snails cannot create soft green algae or diatoms which are the two foods of choice for otos.
Otos are very, very fussy about food and many of them refuse anything but soft green algae and diatoms. They will not eat fissidens or java fern or any other type of plant. Mine will not touch a vegetable and rarely, rarely touch an algae tab or any of the other fish food. A lot of people that keep otos have lots of plants growing so that the otos can suck soft algae and biofilm from the plants leaves.
Is there anyway you can get a 10G tank?