First never mix species. Its a disaster just waiting to happen. Amphibians have porous skin and use toxins to protect themselves from predation. So keeping different species together means they will be absorbing each other's toxins, for which they have no immunity towards.
Second, as Ebony has pointed out, all new animals need to be kept seperated from your established animals. Not all diseases are readily noticable. So a quarnatine period of 30 days is needed.
I would definitely take the surviving frogs to a vet. Where abouts are you? Maybe someone knows a good vet in your area.