Can i give my dog Benadryl and rimadyl?

In general, Rimadyl's side effects are less significant for most dogs as compared to the side effects of Prednisone -- but the biggest risk of problems comes from having both of these drugs in the dog's system at the same time. In general, when changing over from one to the other, we try to aim for a 48 hour "washout period", to allow one drug to clear before introducing the other. If you gave a dose of Rimadyl earlier today, I would not suggest using Prednisone for at least another day and a half, if we're trying to do what's safest.