bigger tank is better, and having sump, even more. had a paired mandarin (dragonet) since sometime middle of last year, was able to train the male to eat pellets, but the female still won't. unfortunately the male succumbed due to an unknown cause - hole in the head (can see my thread poomoon upsized) early this year.
but since then the female is still alive and very healthy. i just have a 2-ft with no sump, so far i don't see her tummy shrinking, just feeding on pods. good thing is she feeds on live brine as well, which fills her tummy fast. when available i also put some tigger pods just as a feast. spending a couple of $ for brine every week allows the pods populate again, and to see her pecking a lot only on one area is a good sign.
it's the same way I had a couple of citron gobies and 1 green clown goby. was able to train 2/3 to eat pellets. however the 3rd disappeared. the remaining citron though also disappeared later. the green goby however has grown since, very healthy, and competes with the other fishes eating pellets. i usually stop the filter and wavemkr when feeding, and once i do this, he comes out of hiding, ready to feed.
one good example though is the spotted mandarin in RD ... it has grown REALLY huge and fat like a pig ! and it only eats pods ...
So bottomline, whatever it is, really need to spend a lot of effort in trying to make them feed ... otherwise, better not keep 'em.
cheers !