There really wasn't a particular order for Pullman cars. What everyone else has mentioned is right on. Observation on the rear, diner in the middle, sleepers on either side of the diner. The second diner probably wouldn't be found on a train of that size, but typically there would be a dining car and a lounge about mid train.
As was already mentioned, a few baggage cars on the head end or express reefers would really enhance your train and provide separation from the locomotive. One other consideration on an all Pullman train would be the crew sleeping quarters commonly referred to as the crew dorm. In the heavyweight era, the dorm would normally have been mid train and be part of the lounge car.
It was not uncommon for the all Pullman trains to be overnight trains while have a daytime train that was all coach. Classic examples include the Santa Fe Super Chief as the all sleeper train vs the El Capitan as an all coach train, the SP Lark & Daylight and the PRR Broadway Limited and Admiral.
Have fun with your train!