i seriously wouldnt recomend gravel or crushed coral. i put it in one of my fowlrs when i first started and you have to vaccum it, and it is jsut a huge mess. it jsut traps particles of food in there and is just overall not a good thing. id get some argonite sand to help with buffering and jsut get a few cierth snails to prevent gas buildup.
as far as fish, there's a large number of different types of fish fish that can go in there:
-any of the small gobies (neons, hector's, yellow watchman, clown, ect)
-some clowns (i wouldnt keep clarkiis, moroons, tomatos, and the other lager species)
-firefish
-damsels and chromis
-grammas
-some pseudochromis sps.
-most other fish that will stay smal, wont get too agressive,l and not swim around too much.
there are some options, but ot awnser your question, yes, you can keep two clowns and a shrimp. lol. IMO, that would be a pretty good stocking size too.