Two words: Seniority List...
They will almost certainly fill the initial seats from qualified pilots...and I would imagine that since the growth of the new carrier isn't exactly going to be instantaneous, they have the time to train from within. The fly past 60 rule in Canada will also keep the old guys in the seats longer as well. AC will need more new pilots though, but I doubt there will be a need for DEC's. Besides, it may take a year to even get a licence from the government to operate, unless it goes on the current Air Canada AOC...which I've read may cause a few difficulties for it to be called a separate carrier. As for West Jet? I think it's a slightly different scenario there, they need a few DEC's to get the ball rolling, and since they have to add a totally new type on the AOC, the first few new hires will have to be management pilots. That's only if no one wants to fill it internally. It's almost certain that it's a down grade for any of the exisitng 737 Captains, and as the new salary hasn't been made public, it could even possibly keep a few FO's from bidding off the 737 to the LHS of the Q400. Keep in mind it's aviation in Canada and stranger things have happened, but I seriously doubt AC will ever hire DECs.