Hello Mmync,
If your going to make the fuge yourself, why not just T off the drain, one end with a valve and spray bar (with the spray bar submerged) and the other end to the sump portion of the filter bypassing the refugium section. This way you can control the flow to your fuge and have more room for sand, critters, and macros. With a well established refugium you shouldn't have any need for bio-balls or any bio-medium for that matter, so why waste the space. If you use carbon make sure to only use it for a few days each month as the carbon will pull out beneficial things as well. Definitely use the carbon if you are keeping soft corals or anything else that will release toxins into the water. Be sure that with a heater you have the Thermostat end facing into the water flow and the heating element end "downwind" so to speak for accurate control of temperature. Hope this is helpful.
Later,
Myk.