just my 2c. if you want to put on some size, then get your carbs from a natural source. mass gainers are a very expensive form of complex carbs. much more cost effective to get a whey powder and eat it with some quick oats. even mixing it with it you should be able to drink it if you let it soak a a while.
As far as eating, seems like you have a fast metabolism, so eat, eat clean, eat often. Its a slow process but if your always increasing your poundage's every week you'l get bigger. stick to your core lifts. squat, bench, dips, deadlifts, chins. and don't train every day.
As far as creatine,it is usefull once everything is set in place and you have a good diet and training well. then its time to add creatine. You body always needs creatine and you can get it ith through natural sources. but a teaspoon post work out will go towards your next one. Remember we can only rebuild our energy (ATP) via creatine, so creatine is an essential ingredient. the more free creatine, the more free creatine phosphate. and that also enables us to use more sugar and fat as those substrates are used in order to get the C (creatine) and the P (phosphate) back together again.