as far as mylar goes: one it is expensive. two it tears easily unless you get 4 or 6 mil and if you do buy 4 or 6 mil, see answer one. three if it gets water spots on it (which it will) it reduces reflectivity by a LOT. and four it is just a pain in the ass to use. i would suggest flat white paint. it is much cheaper. doesn't tear easily and it won't reduce its reflectivity if water spotted. and finally this is just my opinion it helps in reducing heat in small spaces to use flat white paint over mylar. but as i said there are people who will swear by the mylar. now about the fans a y splitter is more of a hassle than is really necessary. what we want to create is positive airflow. the inline fans you are showing are exactly the ones i use. i actually got the 4" and the filter package from the same place. (i like this store for prices). i stick with 4" fans simply because 4" tubing and accessories are commonly used for dryer venting and therefore available at most do it yourself places. that way i can buy all my tubing and such at HD instead of a specialized place. and i think it arouses less suspicion. i am paranoid though. a little more about your tubing. avoid at all costs making any 90 degree bends in your tubing. stick with 45 degrees or less. 90degree bends really fuck up airflow so i would stick with 45 degree bends or less. your ambient temps are high already in the garage. which imo you need outside air coming in so i would use that duct booster fan on that vent you have on the floor. just don't run very much tubing to this. keep that run short. since we have the hot as hell garage you can cut a 4" hole in the top (or two) and run these fans blowing out. filter then fan then tubing. secure this fan to the top using bungee cords on hooks from the ceiling. this will reduce vibration and simplify installation. it will only really stink in the last 6 weeks or so of flowering so i would not install the filter until then as the filter reduces output of fan by about 30%. and it would be good to remember that you need a light tight as well as air tight cab so use caulk on seams inside cab. if the 400 hps has a vented hood and the ballast is kept out of the room you should be able to maintain correct internal temps. wire the hood duct booster fan to the light timer as it doesn't need to run when the light is off. as always a good oscilating fan will to wonders for air movement and humidity control so up in the space wasted above lights mount an osc. fan to the wall. turn it to low or med because it will dry soil of young plants and if there is too much breeze it could wind burn plants. and just while i'm thinking of it i would stick with indica varieties as sativa strains grow much taller.