Amazing starfish!
But- Be VERY Careful. If you want corals, HECK NO to both of them, ESPECIALLY a Red Knob! The Red knob is related to the Crown of Thorns. Beast like Starfish- it destroys coral! If you want GSP (green star polyps) I have them with my Chocolate and they do just fine, because GSP can suck up the green part and shield itself. But, my Choco was able to destroy my Spicy Lemon Favia. Just depends on the coral. The Red Knob is a big no-no. It has a stronger digestion for corals.
Food wise, other than some species of coral, they will mainly graze off algae and meaty fish food that sinks to the starfish. if this does occur in your tank, they could even eat fish that die. You don't necessarily need to worry about food. Just make sure you don't have any mean fish such as an Undulate Trigger- amazing fish, but often kill tankmates and invertebrates. This also applies for other large triggerfish or agressive fish, but your tank is a little to small to keep a triggerfish long term! Shortterm for a trigger might and or would work if you have a larger tank on hand. Not a lot of other species will pick on Starfish. Otherwise, aggressive fish such as COULD eat your starfish instead of them! But, onto the next subject you said- "How to know if they are healthy?"
Even though some aggressive and strong fish CAN pick on Starfish, Red Knobs and Chocolate's are generally really hardy. There's a high chance of them surviving an attack. They can actually lose an arm and safely and completely grow it back! The other day I saw a 3 Legged Choco at my LFS. It was huge. The Starfish you're looking at are generally hardy.
