Can you get the rash all over your body?

Yes. This can be caused by an allergy or by a direct irritation to a drug inside your body. To happen from the outside of your body you would have to have the causative agent ( example=poison ivy) be spread all over your body.
Depends on the rash. Some rashes go all over the body, such as chicken pox and strep scarlet fever. Other rashes can go all over the body, but tend to like being in certain parts of the body, such as eczema. Acne only affects the parts of the body that has sebaceous (skin oil) glands. So, the distribution of a rash usually depends on the diagnosis for that particular rash.