How juvenile is juvenile? I think Omega one mysis are similar in size to Hikari, which are a bit smaller than PE Mysis. Clownfish are usually pigs and can probably scarf down anything within reason, but you might have something else on hand just in case they end up being too large. My clowns were about 1.5" before they started taking Hikari mysis whole, but they were taking the broken bits before that, so...