I've never tried feeding Hikari massivore pellets personally, but as long as your sharks are getting a varied diet rich in iodine, I'm sure it'll be fine to feed on occasion. Mazuri makes a gel specially formulated for elasmobranchs. I would feed this along with plenty of fresh/frozen seafood. It comes in a powder form that you mix with water, and is a lot more economical than most pellets (A 1 kg canister is around $30 on their website). I had to add some chopped, frozen seafood to the mix (wait til the gel is cool to the touch, but not hardened before adding) for the first couple weeks to get mine to eat it. I've spoken to other people who have used this without needing to add anything, so it'll just depend on your sharks. For the sake of convenience, can mix large batches at once, and freeze/thaw as needed. One caveat here... mix outside, or open some windows... Once you add hot water, the smell is pretty rough. You can also get shark/ray tablets on their website. These can just be pushed into thawed seafood. I would do the small tablets for your guys (at least until they grow a bit more).