You can change as much water as you want without hurting anything, providing it's the same temp and ph as the water coming out.
20% a week would be plenty IMO
Crabs are omnivores. They will eat some detritus, uneaten food, algae, possibly some fish poop, BUT, they will always choose meaty foods over the others. anything with claws is a potential predator.
I don't recommend any CUC actually. I tell people that if they like snails and crabs then add a few for looks/diversity, but don't count on them to keep your tank clean. No crab or snail will do the job better than water changes and algae scrapers.
They are not a substitute for good husbandry.
Most people add too many crabs and snails and they kill each other and starve to death. This is why those online stores that sell "clean up crews" say you need to recharge your clean up crew from time to time.