If you play with an imp as main pet you're obliged to pick dark bargain instead of soul link, imp is our pet with the smallest hp pool (only 200k hp), if you play soul link with an imp he'll always die. Also Without the 20% passive damage reduction from soul link you'll be a lot less tanky.
I would never recommend to do 3s with an imp as main pet.
However in 2s you can, petskill did it when he was playing frostmage/demo
Also when playing with an imp, glyph of demon training is mandatory so you lose a glyph slot