In the last couple years I have seen and handled a LOT of fingerling CNBG, RES, GSF and hybrids. When they are tiny it is tough. The GSF stand out pretty well but the others are tough. When they are as small as the one in your hand a person, at least with my eyes, would need to get out a magnifying glass and start counting fin rays and looking very closely for clues. The kind of time you do not have when stocking large numbers of fish not to mention you don't want to kill them all. But as you noted with the larger fish in your batch, it gets much easier as they get some size.
I'm no expert at fish ID. In fact give me a mixed batch of a number of different sunfish and I would do a dismal job of identifying them. But what I have had experience in with my own ponds is having recruitment from a very limited number of variety possibilities. For example when I stocked my sediment pond there were only three possibilities - RES, CNBG or GSF (not stocked) and any potential hybrids thereof. My forage pond had only RES stocked and GSF (not stocked)and produced lots of RES/GSF hybrids (because of the small number of GSF and lack of potential GSF mates so they chose RES males). Having only those four possible choices to choose from I got pretty good at being able to differentiate. I have handled literally several thousand of them from 1" on up. Now if you threw in several other potential species or I went to someone else pond, I would be lost at ID'ing them.
Sounds like you are on your way. Good luck.
