Ok to move a serpae tetra to a 6 gal with 3 white clouds?

Serpaes can have nasty attitudes, even that injured one might start harassing the white clouds. Also the white clouds like cooler water, where as the serpae likes it in the high 70's, which is where it would heal better, not the in the cooler water where it may be more prone to infection due to the decreased metabolism. If you can I would set up a hospital tank with a heater, and nurse the guy back to health.

Serpaes need to be in groups of 6 or more to spread their agressiveness around. I once had a group of 10 or so that got along well because there were so many of them. They even did funky little dances in front of each other, although I never saw any eggs or anything.

You really want that serpae in its own tank to heal up. Perhaps just a large bucket with an airstone and heater (unless you can keep the bucket warm some other way). This will be like a little quarentine tank. 5 gallon buckets are cheap and then add some good meds like Melafix to help the healing process. When he is recovered move him back to the big tank.