The benefits of
flax seed oil are numerous ranging from improving cholesterol levels, cancer prevention, to mood enhancement. For your goal of adding size, flax oil would be key as it is an easy way of adding quality calories to your diet. Pass on the capsules (you would need to take far too many) and go straight for the liquid. It does not taste great but is your best bet per serving. 4-5 tablespoons per day would add an easy 500kcal to your diet.
Creatine & glutamine are excellent choices also. You may want to skip buying straight glutamine and get a
protein powder that has glutamine in it (most do). This way you will get your full range of aminos and add some calories also.
Hope this helps. Good luck!