I wouldnt buy any nerite snails your assassin will eat then and nerite snails need brackish water to breed
Doh, forgot about that, sorry!
Yes, I think shrimp are perfect for your tank. Especially amano shrimp as they are known to be one of the best algae eating shrimp out there, but I'm sure almost any dwarf shrimp (ghost, cherry, red crystal) will get the job done.
Shrimp are really fun to watch as well. Every time I see one run around and eat something I think of Finding Nemo when the shrimp comes out and cleans little Nemo off when he first enters the tank.
+1 for shrimp