They get about 2 inches and you could have quite a few in that tank, depends how much algae you got, i would get like 8 or 10 to start with and then keep getting more in necessary.
EDIT: I see you have clown loaches, not sure but they might try and eat your shrimp, dont take my word for it though as ive never had clown loaches.
I expect the clowns would attempt to eat the shrimp if I know loaches they usually are a little aggressive even peaceful species can be a little agro but shrimps would most likely get eaten I wouldn't risk it Oto's would be the best bet