I have a 15 gallon with a ten gallon sump/ refugium, so maybe I can help you out a little.
I agree with the damsel thing, don't do it. They get kinda large and nasty when they get older ( usually) I have a false percula clown and a Sgt. Major damsel a customer brought to me in an Arby's cup. The Sgt. major started off smaller than the clown and is now almost twice his size. ( He is either going to Tybee Island marine science center or the LFS. )
Anyway, as far as clean up crew I have 3 redleg hermits, 3 blue leg hermits, one orange and black leg hermit, a brittle star, and 5 snails in the display. In the sumpfugium I have one redleg and one mystery hermit (don't know what kind he is) and a banished emerald crab. I would definitly wait on the starfish if you get one at all. You read a lot of starfish horror stories on here with new tanks. I spot feed mine about once a week and keep the SG constant and he/she seems pretty happy.
With two fish, I don't have any problems maitaining water quality. I have a lot of live rock and the refugium helps too. The only additives I use are pH/KH buffer and calcium. I have started using coral accel, but I am unsure about it for the moment. I do 2 gallon water changes every other week and everybody seems to be happy and growing well!
Anyway, every tank is different so things that work for me may not work for you, but good luck. If you want to know anything else, just ask! Oh, the tank is a little over a year old.
Tiff