You want suggestions? we are full of suggestions...or full of something :hubba: First,
do not use duct fans for exausting. They are worthless for exausting air, I know cause I have 3 of them. :doh:
They don't create suction because they are just fans in an opening with room for the air to bypass. A centrifugal fan is the same as is used in a "shop vacume". The active and passive air functions just like the shop vac. the air blowing out of the vacume is being pushed by the fan inside the vacume, which is positive pressure. And the air being sucked into the end of the hose where you suck up trash, that is a passive intake with negative pressure(or vacume). You only need a
good exaust fan on the exaust side at the outlet to suck the warm, stale air out of the room, and at the same time, suck fresh, cool air in through openings at the bottom of the tents(through a DIY contraption that will allow air through while trapping light from going through).
I would also tell you to put the 400w in the same room with the 600w and angle them a bit so that the light being emitted crosses each other to the plants. Then buy you a good T5 fixture that emits 40k+ lumens (or even 2 of them) with the 6500kelvin bulbs(grow bulbs) to put in the veg room. I have tried vegging with just T5, just MH, and both mixed. The best results came with the T5 because the plants didn't stretch much and stayed short and bushy. I haven't used HPS for vegging but I have heard that it allows stretching like the MH light.
I know this will be quite an investment but it will give the best results in the end. If you can't afford it all right now, I would suggest starting with both lights in the big room and start out your first run there so that you can set up that room completely and be able to complete a grow without having to do anything or add anything in the middle of the flowering. By the time you get to the end of the first grow you will should be able to get the T5 fixtures for the veg tent.
You will definitely want to get the fan though, here is a real good fan at a good price. I have bought 3 of these fans from this vendor without a problem. and they ship fast right to your door. hxxp://www.ebay.com/itm/6-INCH-INLINE-FAN-SCRUBBER-vortex-hydroponics-hps-grow?item=390264245764&cmd=ViewItem&_trksid=p5197.m7&_trkparms=algo%3DLVI%26itu%3DUCI%26otn%3D4%26po%3DLVI%26ps%3D63%26clkid%3D7228462147729079372
What you will want to do is connect the exaust end to go outside either through a window or into the attic space(if you have good attic ventilation) then run a hose from the fan to the light, then leave the other end open so that the air in the room will be pulled in through the cool tubes. (you can either use a Y to connect the 2 lights to the fan or you can connect them in series). Then you will want to make a connection to the airspace beneath your house(if possible) to allow the cool, fresh air to be drawn into the flowering space. Once you have your lights for the veg room, you can set up the ventilation to go with that as you will not need the same amount of ventilation for that room since the T5 fixtures don't put out the heat that the HIDs do. If your room where the grow is located is kept relatively cool then you will be able to keep the veg room cool easy enough.
With the cool tubes being ventilated constantly while the lights are on, you should be able to bring them as close as about 6" from the nearest top, but I would keep them at 12" for the 600, and 8" for the 400. This will give them room to grow a couple days without reaching the glass of the tube and burning(if you don't get to them for a couple days).
Once you get past the seed stage and are in the cloning you will be able to pull cuttings every couple weeks and have them rooting and vegging at different times, and going into flower room every couple weeks, and then harvesting 2-3 plants every couple weeks. That is how a my buddy does it and he is constantly pulling a couple ounces every couple weeks.
I know that is a lot to digest. If I aint makin sense just ask.