The length of your veg depends on how big you want your plants to be when they are finished. You can expect the plants to triple in size through the flowering period (if you veg for about a month or a month and a half). If you veg for longer than 2 months it won't quite triple in size but it...
Those tubes are for 6" ducting. One of those with a 6" fan and filter would be fine. Just make sure to put the filter outside so it can help cool the cabinet down (if your only using 1 fan)
funny, I just saw that dry thread and checked your signature out. Looks nice, wish I had friends who grew and didn't mind helping. I don't even let my friends know where I live because they are so damn sketchy.
I imagine that most people would need a carbon filter on the exhaust.
If your using a shitty 4'' ductfan with a filter on the out bound and a shitty 6'' on the inbound your tent will bulge out and any attempt at filtration will be useless.
I would go with the 400watter and paint the inside flat white instead of trying to use glue or some other type of chemical adhesive to stick down the mylar. I would also get nice square pots and really pack them in.
I think you should scrap that halogen light, those plants, and everything else. Restart with a cheap 2' Floro tube and only 1 seed in each pot. Keep the tube on 24/7 until you can get a HPS (like $100). Once you get one switch it to 12/12 and you can get a small crop.
Looks nice, but I have to agree with firsttimeare. The fewer holes in the tubs the better, it might be nice to be able to hold some extra water, but if your plug or hoses fail it's on your floor and you've got some dry plants. Just my .02. Hope you get some nice greenies.