Just heard back from the vet. She said they have congestion and she watched them doing the weird breathing. She believes it's an upper respiratory infection and wants to put them on antibiotics. Her second guess would be that they have gapeworm, but since they don't spend much time outside and haven't been on a pond or around other poultry water that this was unlikely. I hope that whatever it is hasn't spread to the others because then we're playing illness ping pong.
As far as their legs, she said they seem fine. She had them walk around a little. She was more concerned with making sure that they weren't outgrowing their bone structure than a niacin deficiency. I'll continue with the brewer's yeast and watch for improvements. They may already be doing better after getting some rest in their own tub and two days of brewer's yeast in their food. I sure hope this is the case. Thank y'all for your help and encouragement. I'm glad it doesn't appear to be anything too serious. I'll be keeping a close eye on all of them for awhile. Probably forever.