Goldfish are a coldwater species that have high bio-loads and produce copious amounts of waste. Fancy GF such as the ones you have should be fed sinking pellets because eating foods from the surface causes them to swallow too much air which can cause swim bladder problems which they are already prone to.
Guppy's are tropical fish which are warm water fish. They along with other tropical fish need to be kept in suitably heated tanks.
As already stated keeping the injured guppy in a tank with very clean water will eventually allow it to heal. Plus if the GF attacked one they may do so with others so besides all the other reasons given is another for giving them their own tank.