Apple or mystery snails are popular, but poop more than they eat. A nerite is a better option, they are not as large, do not eat live plants, and do not reproduce.
Shrimp are your only other option, but are often snacks. Ghost are common, inexpensive, and depending on being true thosts may be aggressive. Red cherries are colorful, small, and prone to breeding prolifically. Amano shrimp are mid size between the two, and eaymt algae, nay need to have their diet supplemented.
Any other shrimp will likely be unavailable, unsuitable, or just too expensive. Remember, nothing is a substitute for cleaning yourself.
Corys should be in groups of 5, and need a much larger thank than 5g
Personally, I wouldn't put anything in the tank if you are looking for cleaner. Even with a snail, they may clean up unused food or slight algae (a 5g tank would never produce enough to feed a snail) but they poop, A LOT and they do not clean the Bettas poop up. So really, bottom dwellers are the same as adding more fish as far as waste and clean is concerned :) My apple snails are the dirtiest things I have owned lol