Hey Maxed Out,

Although I have not started germinating my seeds for my 1st cabinet grow, I have read a couple of Ed Rosenthal's books and I've been reading over these forums for about a month.

For your intake/exhaust: it is advised that you don't use a fan for the intake; you should instead just have a hole that is slightly smaller than the hole for your exhaust fan, called a passive intake. The fan will draw air through the box. If you have a fan on both, and the air going in > the air going out for some reason as the plants grow, it will force a lot of delightful yet unwanted smell out.
For the CO2, you should not run the fan while the CO2 is being administered. This would suck the CO2-rich air out of the box and waste it. Instead, you should hook everything up to the same controller so you don't have to worry after initial setup. You could also use individual timers for each device, set so the fan is off when CO2 is on.

Hope this helps.