I had a rabbit snail in my 3 gallon with my male, they poop a lot, but they got along very well! now she's in the 29 gallon with two baby snails, and her first baby is still in the 3 gallon. the five gallon with a rabbit snail should work fine, just keep up on the cleaning.
as for algae, my rabbit snails don't seem to do much for mine. their big shells are kind of awkward, and they don't climb the sides a lot. when they do climb, they are able to move pretty well, they just don't do it a lot, and my algae really isn't taken care of (it might be the wrong kind for them but eh). i'd supplement with algae wafers, other sinking wafers, and fresh veggies. My Darroh looooves some cucumber :)
another thing about rabbit snails is that they reproduce sexually. if you have just one you dont have much to worry about. BUT you might wind up with a female, and they can store sperm for quite some times. I bought one rabbit snail, she had a baby a couple weeks later. then i moved her, and a new rabbit snail into the 29, the new one had a baby too. that new girl died, but just recently Darroh had another baby. They have just 1-3 babies at a time, which is nice (mine both had just 1 at a time). So just in case you randomly have tiny babies, that's where they came from!
i'm in love with rabbit snails. you'll love yours too!