It seems like with any group of new frogs, there is always a "skinny guy." Most often, this isn't really a problem, especially with leucs, unless the frog is actually emaciated and appears ill or hides out instead of coming out with the rest to hunt. The skinny guys are often males. The ladies tend to be fatter. I'd watch the eating habits closely. If he is lively and out there eating with the fat gals, and they aren't trampling over him, I'd let it be. Just keep a careful watch. At this point, you should be feeding plenty. Leucs are pretty voracious eating machines, but they do get along rather well in groups without a lot of obvious intimidation.
It's usually a good idea to keep any new froglets together in a small nursery tank for awhile, just to watch them and check on their health and make sure all are eating, and let them grow a bit, before introducing them the the permanent habitat. But since you didn't do this and they are leucomelas, not some delicate thumbnail babies the size of an ant, there's probably no harm done.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus