There is no cross-database I can find for counterindications of vet meds but Rimadyl is an NSAID which are not usually conflicting with the antihistimines. I ran drug.com's database against several common human NSAIDs I could find and no interactions came up. I couldn't find a vet drug interaction data base.
I looks like it would be fine but it's totally a shot in the dark. If you don't have an e-vet close to call, let me know. I have one and can give them a call.