I have kept a variety of corals with seahorses. Leathers, mushrooms, gorgs, ricordias, zoo's, sponges, macro algaes, colt's, sun's, and even montipora SPS. Dwarfs are more limites. Gorgs could be cool, not an expert on the rest. I do know sun's are a no no.
As far as fish and cleanup crew.
I like gobies and have kept firefish with seahorses. i have kept about 15 different gobies all were fine. Peppermint shrimp are cool, but cleaners pose problems IME. I also stay away from other shrimp species. The only crab I use is a scarlet reef hermit, have had problems with many others. I have also had succsess with some blennies, but they are more risky. the tail spot seems to be cool,but IME lawn mower blennies can be very aggressive and are not good tankmates. I have even kept various wrasses with seahorses but only found the 6 line to be O.K. IME clowns even at a very young age are not suitable tankmates.
I have tried several tankmates. When I started there was no defintive list so I had to find my own. I have found somethings to be completely cool, others to be alright, others to be horrible.
For cleanup I like naussarius snails, trochus, snails, turbo snails (provided you don't have pretty red macro's then they suck IME), peppermint shrimp, and a good sand sifting goby.
IMO you will not be able to keep a starfish in a 30g for long.
Good Luck.