Douglas is one I'm hesitant about, but I believe McMann is also a center.
And really, I want to see the Leafs with some beef, and while McMann isn't a monster, he's over 200 lbs., and Douglas is. And I've read McMann is a leader.
Really, to me it's about skating and positioning for their 7-8 minutes.
Yes, I think 7-8 minutes a night, along with practicing with the NHL team is fine for 4th. line rookies.
You are likely correct, in that they might need more AHL time, but heck I think their top 6 will carry the offense, and the 3rd. line with Kampf will be the shutdown. The 4th. line is an investment.