I think William Villeneuve is a more fair comparison for Lavoie personally, which is a pretty solid defenseman. I'm not really surprised however as I'm not sure some of the other top guys were going to commit to Cape Breton anyway.
It seems like a reasonable fit to me, but they will need to get forwards in return for Langlois when they move him in my opinion. They should also look to draft a forward with their other first rounder.
I'd feel very confident in having the following defensemen going forward if I were them:
Daigle - Lavoie
Perron - Shortall
Buteau - X
Nothing wrong with that blue line, you've got size, puck moving ability, physicality, skill and leadership provided by different players.
Their offense really needs a hand though...