Man, that's something else, that picture. Besides it being a great picture, the differences in sizes is really amazing. I've exerienced the same with res's.
For some unknown reason, a few hatchlings are just finicky, or else weaker. I don't really know what the problem is. This guy, however, grew up in a supermarket fish tank, so I don't know the answers to your questions.
But I've had him for two weeks, and for that entire time I don't think he ate. Then, just a few days ago, he started to nibble on some JBL Gammarus. He didn't eat yesterday, or the day before, but just tonight, he half-way dug in! I was amazed.
Yet he still doesn't swallow all his food. He takes minute amounts, and I'm hoping he's eating something. I put raw meat in a little plastic dish, some cut up shrimp; some floating sticks and JBL Gammarus. I also put in a red worm.
I'm letting him float around inside the aquarium within that plastic dish. I'll check on him later and see how he progresses.
I think putting him outside in the sun might've helped. I don't know. I just hope he survives, since there's nothing else I can do.