thats how i got my tent set up. Filter ducting vortex ducting cooltube ducting exhaust. I have a little 80 cfm fan for air intake. Having a stronger air intake fan than your using for exhaust will cause a lack of negative pressure in the room, which is bad for the plants and your grow in general (i think thats right, in fact im pretty sure cuz my shit is workin just fine so far, lol). im using the stronger fan for exhaust, and the 80cfm fan for air intake and its working just fine because i have another one of the exhaust holes open so that i have both a passive air intake plus a semi-active intake, so it works just fine. Youll be straight man.
For veg, go 18/6 or 20/4 . for flowering go 12/12. As far as how long you leave your plant before you flower, i would say personally to leave it for about 5 weeks depending on how big you want your plant. you can leave it veg for longer, but remember that your gonna need double the height of your plant in your growbox when you start flowering. its possible that the plants can double maybe even triple in size.
-hope that helped.