I would suggest using dry aragonite sand, and would pass on the refugium mud since it really is not necessary and many that have tried it wish they had not and saved themselves the headaches.
The best macro algaes for PO4
export is chaetomorpha, grape caulerpa, and i've also heard turf algae is another good one. However, the grape caulerpa will need 24/7 lighting and frequent pruning otherwise it will go asexual and turn your water milky white. It also gets out of control pretty fast, but i guess that wouldn't really be a big deal since no one will be checking out your sump/fuge. I would say your best option is to go for the chaeto and the turf macro algae, but you can throw all kinds in there for maximum nitrate reduction, assuming your willing to keep up with the maintenance that comes along with it. I've never heard about Xenia being an exporter of nitrates but since it has photosynthetic algae within it's body, i guess i could believe it. But most coral are sensitive to high nitrates so i wouldn't just go throwing a stalk of Xenia into dirty nitrate water LOL. As for the lighting, i think it really all depends on how deep your refugium is, and where the refugium is placed(if it is integrated with the sump). If it's just in a corner of the sump, i'm sure you could get away with a PC
clamp-on light, depending on how deep the fuge is as i said before.
The whole point of having a refugium is so things like pods
can copulate without being eaten and then eradicated. So putting fish in the fuge that eat those bugs wouldn't exactly be a smart idea, assuming i understood the last part of your post correctly.