Perhaps kitty #1 has worms or some sort of parasite. If his belly is swollen, it sounds like he might have worms, and you'd probably need to take him to a vet to get some medication.
As for the poor little guy with the bad legs, maybe if you tried some physical therapy on him? Will they let you hold them? I'd take him to a vet too and make sure his little legs aren't broken. Perhaps his muscles just didn't have a chance to develop at the same rate as the others. If he's the runt he might have been pushed to one side early on. Helping him get some movement and exercise might help.