for snail and shrimp yes...

saw some color snails in planted tank when i visited sg lfs. 88 display tank which is inside the shop one...also hv few. heard this type of snail won't spawning like the apple snail. they do clean the surface of tank and leaf.

shrimp...like yamato, malayan/malaysian, cherry..etc shrimp do help to clean the algae. Some of the ppl setting up the planted tank just keeping with shrimp only. no fish.

Crab...never see in planted tank. may be u can try. :P