It depends on mandarin, but yes I’ve seen some come up and eating pellets that are floating on the surface although that’s rare :)
As for food, you definitely should use sinking pellets that are small enough for mandarin’s mouth. I use Hikari’s megabytes red and Otohime b2 (those are from Japan but I’m sure you can get in the states or something similar easily).
The best way to get them to eat is put them in a small isolation box (you don’t want too big as you want to increase the chance of mandarin finding pellets) and put live brine shrimp, and pellets together. As they start eating brines, they’ll accidentally eat pellets on the bottom, and they’d start eating. If you do that for two weeks, I think most of the time they’ll be eating pellets by then. I have 8 dragonets and I had trained all of them to eat this way! Also it’s a lot easier if you have another dragonet eating pellets, they’ll start imitating. That’s why I used my spotted and green mandarin as props (they already eat pellets) to get the new ruby dragonets eat and they were eating pellets by day 2
If you use auto feeder, I would just make sure that surface has enough wave, so that pellets would naturally sink