Starboard question..

[Delphinus]

To me, keeping a mandarin is more dependent on size of system (or amount of live rock, depending on how you look at it, but these two variables are roughly linearly related), and whether the fish adapts to being fed, and how much competition there is.

The population of pods surely is larger with a sandy or rubble substrate but I think relying on the tank to self-sufficiently feed the mandarin is likely doomed, since pod populations tend to come and go anyhow.

One trick you can try, get some coral rubble pieces and pile them up. This technique is called "pod piles" (if you want to do a search), but the basic idea is that it gives the pods a spot to hang out and theoretically escape predation therefore increasing their numbers. A sort of in-tank refugium, so to speak.