CO2 usually has a relationship with the pH and kH of the water, this is how drop checkers work. I've never used one, if the fish look like they're gasping for air at the surface, there's likely too much CO2.
HTH
I haven't tried them but there has been a few discussions on them. I believe that if I were to ever have one, I would plant the sump and rotate the Bettas daily between their compartment and the larger tank. Any fish kept in a restricted environment such as that is going to be less active. I would also put a bit of java fern and/or moss in each compartment to look nice too.
JMHO