I know how it goes. This high metabolism is like a plague to us skinny folks, while everybody else is envious of it. Do you know how many calories you are eating a day to begin with? Or have a ball park idea about how much you eat per meal? Im sure you know about the 5-6 meals a day....... Instead of dishing out suggestions Ill suggest you to invest in a book. I was introduced to the book "Scrawny to Brawny" by Michael Mejia/John Berardi. In it they have details on workout routines and a hefty section on Nutrition. It helps some people to have literature in their hands... If anything just go and browse through it at the store.
As for the taking of the weight gainer, you can always use it as 1 or 2 of the "mini" meals during the mid day/afternoons. Sometimes i split the portions in half and just add more milk and maybe an extra scoop of just plain whey protein(it tastes better and lasts abit longer, specially if you do it 2wice a day.)
Taking it before working out can be a bit iffy. I personally like a bit of substance for energy before working out but i know a lot of people don't. You could take maybe half or quarter of the recommended amount before the gym. Or even a simple PBJ and a glass of milk could suffice. Or just go with the other mixes that you said you're already taking. I think i just rambled on about alot of different things but definitely check out the book.
One last thing, look into Casein. Its a slower digesting protein and many people take it in addition to whey or soy. You can take it before bed(if you dont mind that full feeling before falling sleep) and that will help feed the growing muscles during the night.