I would feed the entire flock a starter, grower, or gamebird feed until the Sebrights reach laying age. Make sure you offer some oystershell or crushed eggshell free choice, too, so that any laying hens can get the extra calcium they need. You definitely don't want to feed layer feed, since it contains too much calcium for young birds.
They may eat a little of the oystershell, but not much. Chickens seem to know when their bodies need extra calcium. If they need it, they'll eat the oystershell. Otherwise, they should leave it alone.Thanks! Won't the Sebrights eat the oyster shell too though? Just wondering. Thanks for your help, it is very much appreciated!