Its probably better for the clown when they dont end up in the middle of an ammonia spike due to a dying anenome.
Anyway-
Clownfish: go for the smallest two you can find and they will form a pair bond.
Cleanup: I personally dont particularly like hordes of hermit crabs, so would just go for 3 or so of one of the smaller species. Add a load of small snails and let whatever comes out of your live rock do the rest.
Other fish: Maybe a neon goby and some sort of rock dweller eg. royal gramma.
If you are looking at keeping corals then it would be a good time to think about lighting, either a single 150 watt metal halide if you are looking to keep clams or hard corals, or some sort of power compact or t5 lighting for soft corals and easier to keep hard corals.
Filtration- generally mechanical filtration has been replaced by live rock as the main filtration and as long as stocking is sensible then this is surficient for your tank. I do however use a cannister filter for circulation and as an easy place to run carbon but this isnt essential.