You don't need a filter in that tank. I don't use one in my breeding setups. All you really need to do is make sure the water stays between 40 and mid 80's and the shrimp will stay alive. Keep the temp between 60's and low 80's and they will breed if you have males and females. I successfully keep over 300 RCS in my 10g breeding setups. I'd say you could keep about 75 in your 2.5g tank. I wouldn't start with more than 20-24 of them though. They eat algae and biofilm along with flake foods, algae wafers, pellet foods, dried up leaves, frozen foods like bloodworms, brine shrimp, krill, etc. They will eat basically anything you put into the tank. You only need to feed them a couple of times per week as they mainly live off of the small microorganisms/algae (biofilm) in the tank.
