I'm glad your tanks are at least 2.5 gallons. ��
Otocinclus would be a great addition to the tank, if your betta is okay with them. They're really good at eating algae. Only one might be able to be added as they do have a bio load.
Nerite snails will devour your green algae and they're also pretty cool looking. Their shells will give them protection from your betta and they eat all types of algae except black beard and blue green.
Amano shrimp are the ones you want if you want a shrimp who's constantly trying to clean your tank. They love algae, and can survive only on algae, but will go nuts when you feed your betta. They'll fight each other over the leftover food on the ground and search tirelessly for all the food. Although, they don't have anything to scratch or bite with so they can't hurt each other. You can probably have five in there and you still won't notice any change in your bio load. They're bigger so your betta may leave them alone. They don't like temperature of 79 or above though.
If your betta will allow, go for cherry shrimp. Very beautiful, and love eating. Like all shrimp, they have negligent bio load so you can have ten or more in there. They do breed in fresh water, and the shrimplets wouldn't really survive your bettas' cravings, but the adults will be safe from getting eaten. They're also very sensitive to temperatures above 79.
I highly recommend getting a piece of Malaysian driftwood for each tank, with some Java moss glued to it, if you want to go the shrimp way. Also get some almond leaves. The tannins from them will soften the water and release stuff that the shrimp as well as your betta will love.
Here's one that will definitely work but I wouldn't try it as my first choice. Ghost shrimp. They're underrated and usually thought of as food for bigger fish. These guys are scavengers and won't touch your algae as long as they can steal food from your betta. They are, however, great at cleaning up the leftover food that went on the floor. They also don't require the addition of wood, moss, and leaves. But a word of caution, they can bite! They might "taste" your betta to see if it's dead.