Flying LGA-YUL later this week in J on a CR9.
Does AC do plane side valet bag check, since I would expect the overheads not to fit all carryons?
For a CR9 - almost certainly not. They will offer to check bags through to the destination at the gate just like the majority of flights these days but to "skycheck" the bags directly at the plane isn't something they do for a CR9 (if memory serves only the smaller original CRJ's and the original Dash8-100 and 300 ever offered that).