This is an easy decision IMO. You immediately bring up Hamilton, put him next to Seidenberg on the second pairing, and if and when he has growing pains you replace him with Ference on defensive zone FOs. Play him on the PP in practice as much as possible and give him some shifts right away (because frankly, he comes immediately possessing the best offensive tool set of any of our other D men). You limit his ice time enough where he develops but shelter him as much as you can.
If he struggles right out of the gate, you have Krug to step in should Hamilton need to see some ice time from the pressbox.
Hamilton doesnt need any more time in Juniors, he needs to be around Chara, Bergeron and Thornton.