I'm starting up a system of my own and I have similar questions... but they involve CO2 injection in a very small (cabinet) setup.

From what I've been told, CO2 can be applied from root to two weeks into flowering. That seems to hold up with all the experts I've asked.

However, you cannot have your exhaust fans running while your CO2 system is injecting CO2 into your grow room because the fans will just suck the gas out and keep the rooms CO2 levels at normal (about 400ppm at sea level.... your plants like it at 1500ppm).

So, your ventilation system must be off during CO2 injection. Plants in a CO2 rich environment, however, can sustain healthy growth in much higher temperatures (up to 90-93 degrees from what I've heard... but that sounds too hot). If temperature is too much of an issue, you could put a small, closed-loop A/C unit running on the same timer as your CO2 system, and then set the timers on your ventilation system to go off when the CO2 goes on. That way... your room should stay at pretty much the same temperature even when you're injecting CO2 and the vent. system is off.