If the canisters have pre filters on them would that not be amazing feeding places, not to mention the rest of the entire tank, substrate , plants, moss, wood, feeding dishes , just to name a few. To out gas CO2 & jack up ph values does not make sense.
Canister filters outperform sponge by like 10 to 1 in flow & filtration, & the shrimp can still graze on the pre filter.
I guess in tiny 2 -5 gallon tanks you would not have much choice, but larger tanks over 5 gal. would benefit from canisters. With NO excel usage!
I guess the cheap way would be with sponges.