Got a good friend that picked up a longhorn cowfish... Named him Emoji

Thing was maybe 1/2" across when he came home. That was 6 months ago. Put him in a 29g tank, with a pair of fairly aggressive cinnamon clowns. Had to target feed him at first, but these days, he competes just fine. He's probably 3" long, horns growing in nicely. Happy, healthy, and growing rapidly.
When he gets big enough, he'll be moved into a 210, but he needs to be large enough to deal with the other inhabitants... a dinner plate sized koran angel, football sized spiny puffer, couple tangs, and a dottyback that's about half a pound.
Needless to say... at 1/2" in size, he'd not have lasted long in that tank. I don't see ANYTHING wrong with putting a juvenile fish in a smaller grow out tank while they're getting big and strong.