Taking russell over corrado would be terrible.
They filled the veteran quota with polak.
Russell would be a significant upgrade over Corrado. Frankie is a bubble guy. If they don't sign one of the mentioned names I'm sure they'd have no issue going into the season with him and continuing his development with the Leafs.
But, those 3 vets would help the Leafs be better than they would be with Corrado in the short term. Cheap 1 year deals on Quincey or Wiz could be sold for top dollar at the deadline. Russell on a 2 year deal is a Bozak-like veteran that could be shopped or kept for another year.
I think the likely scenario if they were to get one is Frankie starts the year as the #9 and goes unclaimed on waivers, he is used as the vet top pairing RHD on the Marlies for half the season, as Justin Holl is the only other RHD on the team. The other half the year he sits in the pressbox in the NHL as the first callup on D, as the rest of the Marlies D have some developing to do. Get's in about 10-15 games through injuries and what not. Then, takes over as the #6-#7 to close out the season, after Hunwick and Polak and potentially the other signed defender start being moved out for futures.
A guy like Dick Panik was claimed and had a fairly decent season in Toronto, a better one than Corrado. He cleared waivers to start the next season and was quickly moved out. If the Leafs make any more moves that involve adding to their skaters at the NHL level, Corrado is on the hot seat.