I'm not sure of the exact relationship between CO2 and O2 in the water, but I don't believe they are mutually exclusive.It seems like most people on the high-tech side of things aim for around 1.0 drop in their pH by the time the lights turn on. This should give you a ballpark CO2 level of about 30ppm. Because CO2 is so cheap, I want to push it closer to the limits. I've found that at around a 1.3 drop in pH my fish are gasping for air at the surface. So I see a 1.2 pH drop as being a happy medium between pushing the plants close to their limits without stressing out my fish too much. So I'm not aiming for any specific pH level per se, I'm more aiming for the specific drop.
My pH drop is from 8.1 -> 6.6.
I found that to keep the fish from labored breathing in the morning, I run a venturi through the night that aerates the water. I figure this is also good for the plants.
Maybe an expert can correct my thinking, but I believe with proper aeration you can run high levels of CO2. That said, when I had alot of surface exchange (HOB filters), there was more aeration taking place (didn't need the venturi), but CO2 would gas off much quicker.
I think with some more aeration, you can keep CO2 levels high without hurting your fish.