[Booger]

All my old tanks have used canister filters but I am thinking of using a overflow that will run into a fw refugium that will include a reverse flow ug filter.

Don't do this. Aside from maybe a specialized shrimp setup, UG filters have no place in planted aquariums. Keep your sump as simple and maintenance friendly as possible.

Will all this circulation and generating the co2 in the refugium effect the co2 levels in the main tank.

You will use marginally more CO2 to achieve 30-40ppm in the main display, but probably not much. You can cover the sump to reduce CO2 loss, but I would suggest this more for noise and evaporation control.

Also I would like to do a moss wall but am a little worried about the moss getting sucked into the overflow and clogging it. Any thoughts?

You will notice that most tanks have overflows that are a tiny bit shorter than the rest of panels (i.e. even if the overflow teeth are completely clogged, water will run up and over the teeth without spilling onto the floor). Plant matter does constantly get caught in the overflow teeth, though. There are a few different ways to address this. Check the reef forums.