Yeah he pretty much picked the worst team to sign with. Calvillo is an iron man and superstar, he'll have a hard time getting any live snaps. Like someone said above, the only teams where he could get some playing time are Winnipeg and maybe TO behind an aging Allen (he's got to retire sooner or later).
But by no means would he destroy the league. The CFL has had better than average mobile QBs come here before and do next to nothing, either not becoming accustomed to the style of game, or just not fitting in. Carter is by no means the most skilled QB to ever play up here, let alone the most skilled QB playing right now.
Personally I think it would be good for the league if he did well though, and managed to get back into the NFL as a result. More players who couldnt quite cut it but had tremendous skill and potential (as Carter does) might give the CFL a shot. As it stands plenty of great college players who are over looked by NFL scouts for whatever reason are primarily who fill the skill positions up here.