Well here we go...

To get the old coralline off, try soaking them in a moderate white vinegar - water solution. If you can run the skimmers so the solution is circulating in them, they'll likely clean up more easily and quickly. Anything that doesn't come off on it's own during the soaking can usually be cleaned off with a toothbrush and a little scrubbing.

Just a suggestion...
If you're planning on sand in the refugium area (kind of guessing so from the diagram), there's really not much need for it. Even if you wanted to put a DSB in there, it's such a small area / volume of sand that it's not going to provide a noticeable impact in terms of anaerobic bacteria breaking down nitrates.

I've seen just a few recommended sizes in different places, of course in places that I can't seem to find links for at the moment (links hopefully saved on the home computer). But, if I recall correctly, the tendancy is for the smallest effective area to be around 600 square inches when a mimimum recommended size is mentioned. This value should definitely go up as total system volume goes up, although I'm not aware of anyplace that says exactly how big a DSB should be for a system of x gallons.

Here's a link I found that goes into it a little:
DSB FAQ
There's also some info on WWM that talks about DSB size, but there's general recommendations there that some DSB is better than no DSB if someone wants a DSB.

I guess it's a matter of what each person wants in order for it to be effective. Any size DSB will support a certain number and type of sand bed critters, but I find it very hard to believe that a DSB that's a part of an average sump / refugium combination (where the refugium is at most 1/2 of the total sump and the sump is around 1/3 of the display tank volume) can eliminate nitrate issues in the average system with a moderate bio-load. It's my experience from working in a lab where others were testing nitrate to N2 gas conversion rate potentials in marine sediments that the process is slow. It's not something that's measured in a few hours, it's more on the order of a few days. The slower the process, the more surface area that is needed to ultimately control nitrate levels and keep them at 0 ppm. This is why I believe a DSB as a refugium chamber in a sump like the one pictured above has little benefits in terms of completely eliminating nitrates. I could certainly be wrong about this, but I haven't seen enough info to convince me that I am. I'd honestly love to be proven wrong -- it would help save me a ton a money on sand.

I run a 5g bucket RDSB on a 50g and it works great, but it's my understanding that each 5g RDSB can handle around 100 - 150g. I wouldn't think that one would be completely effective on a 200g, but if it's lightly stocked and not overfed, I suppose it's possible.