I am not a starfish brain, But I would say with the two you have now you are pushing it.
Starfish for the most part have a very strict and finicky diet. And even though your tank has been up for a year, It is still only a 90 gallon. I would think that the one starfish would deplete that food source fairly quickly, in a few months. After that it would more than likely starve. All starfish are fairly hard to keep because of their specific diets, and basis lack of knowledge of how best to care for them.