If kitty has a hurt foot, bring it to the vet, they will tell you what to do and how much of any meds to give. Don't give kitty any meds even over the counter before you talk to a vet. I don't know what Pen-G is.
Since antibiotics are normally prescribed based on the specific bacteria involved and the accurate weight of the cat, it would be best to consult with your vet to 1) have him assess the injury, 2) weigh your cat accurately and 3) prescribe a medication specifically for the type of bacteria infecting the wound.
You shouldn't give your cat PenG unless you know it's a bacterial thing and not something else. If your cat has an abscess all the PenG in the world isn't going to help unless you open it, drain it, and keep it clean along with the PenG.