There is no definite answer to that question except to say that whatever you purchase must have enough food to sustain it. the reality is you really don't need X amount of crabs and snails. For "cleaning up", nothing beats a mag float and a water change.
excess algae can be controlled by keeping the nutrient level low.
uneaten food and fish poop on the sand bed can be controlled by using power heads to increase flow so that nothing gets a chance to settle on the bottom before your filtration can remove it.
If you want some of these to have diversity in your tank I would add them a few at a time and see how that goes, before buying one of those packaged suggested CUC's which are in many cases way more critters than your tank can feed.