McAdam should see a lot of ECHL action likely.
I have to think that is the most likely case, at least the first month or two, until an injury occurs and either necessitates a call up to the NHL (presumably Gibson) or to the AHL (with McAdam being called up).
However, if McAdam shows well, something will need to be done to clear a spot for him in the AHL. A month or two in the ECHL is OK but, ultimately, the Isles need to get him minutes with the BST if they want to see him progress and develop.
It is a good problem to have, you can never have enough lottery tickets, and goalie prospects are like pitching prospects in MLB, you simply never know what will happen year to year with their development, growth and health.