As far as i know, waterlife protozin meds are not adviseable to use with other meds, however i believe this is to help prevent the tank being overdosed with various med ingredients, as far as i am aware melafix has different ingredients to waterlife protozin so might be ok to combine the two meds.
From the symptoms you are describing though, i would say that your fish have columnaris, which is an external bacterial and fungal infection, personally i would use "anti fungus and finrot" by Interpet and Pimafix (these two meds are totally safe with each other).