The best bet for this size tank is either a brittle/serpentstar or a Linckia multiflora. Most other reef safe stars get too large for this size system. While brittle/serpentstars can be spot fed, the reef safe seastars rely on lots of healthy mature LR and can not be spot fed. They are also very sensitive to fluctuations in water parameters, making life in the smaller tanks a bit tricky.
Linckia multiflora is a smaller, tougher star than most. While it still feeds on LR (add as much as you can), it has actually been reported as a hitch hiker on LR, meaning it is much tougher than others. It has also reproduced in these smaller systems. I believe Ryebread has one in his 29g which has reproduced (which it does asexually through arm drops). You can do a search for it.
As mentioned, the other reef safe stars get too large, or are generally too delicate for this size tank. The tend to starve to death even in much larger systems. Avoid also the small Fromia sp. stars. Because of their small size, they are often placed in small tanks...however, their diet is open to debate and it appears they may be even more picky than other Linckia, perhaps feeding primarily on encrusting sponges.
Brittle/serpentstars will also be suitable, but may hide too much or generally be a bit bland for your liking. Whatever you do, avoid the green brittle, a known predator in the wild (not always in captivity, but a risk)
All of these guys will require pretty long acclimations for best success, 4+ hours using a drip method....but sounds like that is some time down the road!!
