Well, you are going to gain fat when on a mass building program. But as for what will put on the least amount of fat, I would say have the carbs before yoru workout. This way you will be able to workout harder, and you will burn off the majority of the carbs you put in. After your workout you would be using the carbs to replenish muscle and they woln't be burned off as much. All in all, don't worry about putting on some fat, your trying to build muscle, you can focus on shredding that fat when you cut up.