Actually, for Super Suns or Suns, it doesn't matter how many heads they have. You just have to feed one head if all the heads are joined together and that will be sufficient. Typically for my suns and supersuns, I just use Henry's Reef Gourmet (HRG), mix with water, suck the mix into a 50ml syringe and then blast them over the sun and supersuns. The tentacles will extend and grab the food.
As for Rhizos, they are easier to feed. I don't feed them with HRG. I buy the silver fish from supermarkets and then just place 1 fish into each Rhizo. 50g of the fish can last me a couple months already...and it doesn't pollute the water as much as HRG.