On the occupancy, best to contact the hotel directly. The room info on the Fairmont website does show the "capacity" of the rooms, and most of the normal rooms like Gold Lakeview do have a capacity listed of 2 adults. Personally I think it would be quite cramped with 4 adults and an infant in a room like a Gold Lakeview room, as I don't even think there would be enough chairs in the room for everyone to sit down on a chair at the same time.

On the upgrade question, Accor is not anything like Marriott when it comes to upgrades. As a Marriott Titanium, I routinely book the base room and often end up with a larger suite, etc via upgrades. Accor is nothing like that. Even at Gold/Platinum, the upgrade benefit is a one-category upgrade subject to availability, and the CLL makes specific note that this doesn't apply within the Fairmont Gold floor (e.g. don't expect an upgrade to Fairmont Gold, or to a higher category within Fairmont Gold). It can happen, but it's not routine. Without status, free upgrades would be rare at the CLL. They do sometimes offer "standby" paid upgrades after booking, so that may be your best bet for upgrade options. The CLL seems to have been quite busy this year with the quick rebound in travel especially to mountain locations, so upper room categories seem to be often sold out. But I'd make sure you book a room you're comfortable with and don't expect an upgrade.

I've stayed multiple times at both the Rimrock and CLL and both are fantastic properties, so I think you'll be happy either way.