A newborn motoro, approximately 4", may reach 12" in the first year of life. I have never kept any of my male pups that long but, I would expect that they grow slightly slower.
Live blackworms are the best foods for newly acquired, juvenile rays. You can easily move into earthworm, ghost shrimp, crayfish, frozen marker smelt and frozen market shrimp. Cut the frozen stuff into small pieces with a large pair of kitchen shears and the rays will go nuts.
Getting them off of live and onto frozen can be a little tricky with some rays but well worth the effort.
Colin