If you are talking about a mail/express train, it would have been a hodgepodge of assorted styles. Box cars for mail storage and express cars, all with high speed trucks as well as various style baggage cars. The train may, or may not, include an RPO. Many of these trains may have had a coach or combine on the rear and, maybe, a caboose.
If you are looking to add express and/or an RPO to a passenger train, initially the baggage and RPO may have been ordered at the same time as the cars, so they would match. After photos and some run time, many trains would succumb to everyday railroad life and gradually get a mix of available equipment. Notable exception was the 20th Century which was meticulously keep in original equipment.
It's your railroad; do what makes you happy.