I would worry more about flow and good oxygen in the water than feeding for the first few days. I know it's hard to resist the urge to "see if it will take this piece of food", I can't really do it either. I just moved one of mine on the weekend from one tank to another, it was a bit hard on the anemone and I promised myself I would just "leave it alone for a day"... of course I was trying to feed it that night already. So I'm not very good at following my own advice.
If you can get your hands on some mysis I would try feeding him that instead of a silverside at this point. My main worry about feeding is that if fed a piece of food too large, it could introduce some risk in that the food may decompose faster than it can be digested. If it does eat some shrimp though that is a promising sign.
It's so touch and go for the first little bit. It's hard to know what to do. That first week is just so stressful. I know what you're going through and I wish I could offer something more tangible in terms of advice .. But I think the best you can do at this point is just do the best you can do, and hope for the best (sorry I know that sounds kinda corny).
If he is shrinking at night ... hmmm. This is just speculation now on my part, but I wonder if it could have something to do with oxygen in the water. Typically during "lights on" periods, because of photosynthesis, I would expect the O2 level to be much higher than during "lights out." Could there be something to that? I honestly don't know, but wonder if it would be worthwhile trying something like an air bubbler, or a powerhead with a venturi, to inject bubbles into the water (if not continuously then maybe on a timer to do it periodically, like for a minute once an hour or something) during the night period. If it were me, that's something I might try.
Some shrinkage is of course normal for night periods, though. Mine do "close up and pull in" during the night, and then expand out again during the day. Soo.. it could be just a normal thing, I'm not sure. That first week or so is really a tough slog, you won't really know if you'll make it until you actually get there.
Best of luck!
