True percula clowns will sometimes host in E. quadricolor. I don't think it's a natural host for them; unfortunately, the only natural hosts are H. crispa, H. magnifica, and S. gigantea. The common carpet anemone, S. haddoni, is not a typical host for perculas. All of these anemones are extremely difficult to keep alive for more than a few months in an aquarium. (Less than 5% survival rate after a year) But, lots of people have reported success in getting true percs to host in E. quad. And, as you undoubtedly know, you can get tank bred E. quads.

As far as the mandarin goes, you might have better luck with the spotted mandarin in terms of feeding it frozen food. In your initial post that I replied to earlier, you made no mention of feeding your fish frozen/flake food. That's why I replied as I did.

A 30 hex will not work well for a DSB; it lacks the space for a strong enough population of sand bed animals to perform the denitrification duties of a true DSB. And the shape limits the surface area of the sand, and thus, the sand/water interaction. For a refugium it's fine, and as long as your stocking is conservative your caulerpa growth will probably keep your nitrates manageable, especially if you have a good skimmer removing waste before it goes through the nitrogen cycle.