Idea you need at least pair of oto in the tank to keep them happy. I only have 1 oto in my do!aqua 45p shrimp tank (8.9G) and it is fat and healthy, I guess that due to I also have army of 11 pygmy corys in the tank as well. I guess cat fish stand together.
If you want something to eat up the algae, 1 nerite snail should be enough for 5G. (If you have more than 1 with different sex, you may have eggs all over the place.) Amano shrimp may not be good idea since CPD is smaller than full grown amano. There was a video of amano shrimp attack rummy nose tetra on the forum. Cherry shrimps should be fine but that they aren't that great of algae eater compare to amano shrimps.