I'd recommend using CopperSafe for curing Velvet, but only use a half-dose. I've used this safely with my clown loaches to cure ich in the past, and it works very well. Be patient, velvet will take a few days to a week to completely cure, and you may still lose some fish if they are too far along.
Taking into account the size of your tank, you may want to set up a quarrantine tank for your ill fish. This will cut down on the amount of medication required, and will help prevent the further spread of the disease (if the other fish are healthy and aren't symptomatic).
Also, as centralharbor said (and I neglected to list in my previous reply), make sure there is adequate surface agitation in the tank. Although I think the difficulty breathing is likely because the temperature was too high (which results in reduced oxygen in the water), this will help increase the oxygen levels in the tank and make it easier for the fish to breathe.
Keep us posted on your treatment and progress. Good luck!
Mike