In my opinion, free weights will help you gain more muscle and more weight than machines, as you develop your smaller, stabilizer muscles when using free weights. But machines are also helpful, and I use them in my weight training workout. As far as how often to work each muscle group, my workout looks like this.
Monday: Chest and Biceps (4-5 lifts that target each muscle, so 8-10 total lifts)
Tuesday: Back and Triceps (same amount of lifts)
Wednesday: Legs and Shoulders (same amount of lifts)
Thursday: Chest and Biceps (same amount of lifts)
Each week, I change which muscle groups get lifted twice in that week. Make sure that you do the same muscles on Monday and Thursday, so that they have adequate time to rest.
As far as supplements go, I have had good results with MMA Fighter's Fuel
. It's a new product aimed directly at MMA fighters, and works well for increasing pumps and building muscle faster.
Your weight gain goal is possible, though twenty pounds is a lot to put on in five months. Whether or not you achieve that goal will depend on a lot of things, including your body type. For example, I am short and stalky, and tend to build muscle very quickly.
Anyway, hope this helps, good luck with your weight training.