Dont know where to start on this one. First off, 7x7 with 2 600s is not that much light for that space. I run 2 1000watt lights in a 4x8. I agree the Y is a good idea, but it sounds complicated. Speaking from my personal experience, in my opinion its not necessary. Use reflectors with 8 inch ducting and get a good 8 inch fan, you will be very happy with that setup. You can get away with 6 inch and save a little money, but come on, if your going to do it, do it right. Unless your drawing air from inside your growroom and blowing it through your lights you dont need a carbon filter on the end. Think about it, non smelly air from outside your growspace doesnt become smelly while running through a seeled light. Your carbon filter goes on the end of your other fan that is venting your room. You need to exchange the air in your room every 3 to 4 minutes. No offense to rusty, but he is a little old school when it comes to growing. Also, i would reccomend the good sealed hoods, with rubber gaskets and all that good stuff. I got the cheep ones, like a whole 10 dollers cheaper, and they whistle. Its annoying.