Whats the problem with changing some water in a 2.5g? With just some shrimp, you could let it go for a while, but changing water does more than just get rid of built-up nasties. With no filter or anything else for water movement, you might also have a problem with lack of oxygen for the shrimp.
Key thing wrong with your plan is that most bottom feeders don't eat waste, i.e. poop. They "bottom feed" on whatever actual food settles to the bottom.