Sounds good and depending on what else you put in the tank, you can double that if you feed. In OhToo's 60, I have two serpents, one brittle and two thorny's but no other clean up crew to speak of. We do feed Cyclop-eze daily to the tank and make sure everyone has enough to eat. I think you will find you enjoy the thorny's more than any of the others because they are so bright and so active. I know the fromia are written as THE recommended starfish for an aquarium but I have tried several in different, stable aquariums and only succeeded in keeping them alive a month longer than the pet store. I have never lost a thorny and only a couple of serpents (I have many scattered about in my 7 tanks and only the red have given me a problem and only after two or more years) Wait until your tank is fully cycled before adding the seastars though, don't make the mistake of thinking they are like snails. They cannot take harsh changes, especially in PH and will fall victim to new tank syndrome if the tank is not stable. They are not aggressive towards conspecies (or any others) so they can be added at any time after the tank is mature.
One exception is the green serpent that looks somewhat like a cross between a serpent and a brittle. They have a reputation for becoming aggressive when they get older and can get huge. I have had one for many years (the oldest critter I have) and he has not caused problems but Mr. Green Jeans was removed from OhToo's tank because he seemed to be intimidating the octopus. He had never been a problem (in fact he kept loosing arms and was continually being quaranteened for his own safety) but now that he is growing and maintaing his arms, we felt better safe than sorry.