What dog shampoo do you use for a 6 yr old toy size dog? and how often do you shower him/her?

and is purina anti-flea shampoo dangerous to dogs? my toy size dog had some fleas and the doctor gave him comfortis tablets and I cleaned all carpets, rugs and dog beds and any soft material my dog has been on with water and soap because the vet told me to...the shampoo bottle says that it also helps irritated skin, and the vet said to give my toy sized dog a weekly shower