A few random thoughts:

Maybe experementing with DSBs would be more easily accomplished in your Fuge?
Agree w/Matt on running your skimmer out of the same sump chamber, and his comments on Ca Reactors, although my experience here is only beginning.
With a good setup excess CO2 shouldn't be a problem, but the best defense will be good water movement at the surface for gas exchange.
Your UV should be feed fairly "clean" water, after the skimmer if possible. Then again, water doesn't really flow from one piece of equipment to the other, mostly in/out of the sump from various stages of "Cleanliness".

In general it takes some careful planning. Pumping massive amounts of water in & out of a sump from various locations can result in some surprising water levels. I think I have a very complex filter/sump system which I planned & Re-planned many times - and I still had a "awe ****" or two I had to fix.

- Steve