In my experience, what usually happens is no matter what i'm doing exercise, the more I eat, the better gains I get. now granted.. I was on the skinny end.. going from 135/140 to 165/170. The gains, i'd say, are mostly muscle... hard gained.
And no..you can't avoid getting fat in my experience. That was actually what held me back for awhile..I was afraid to really pack on the calories. It's no big deal though.. you just add the muscle, then do a week or 2 of cardio and you're set...