I have used melafix a few times with no problems and then I mixed it in with Liquisil , A dechlorinator and a stress coat when I was doing a water change. Pre mixed all the chemicals in the bucket and added them slowly to the tank.
Bombed my tank and I had to do an emergency 75% water change - lost quite a few fish - now i'd added the other 3 together every time i did a water change with no problems but thsi was the first time i'd added melafix into the equation - so I would say be careful and add the additives at different times over the space of a couple of hours to be on the safe side