Alex
The E-8s first came to the PRR probably in 1950-51. They were 5 stripers at that time i believe, with the PRR going over to the single stripe in the mid to later 1950s. These locomotives would be assigned to the best PRR long distance trains like the Broadway Limited, The General , Spirit of St Louis, etc. The cars in these trains would be the lightweight streamlined tuscan red cars and include diners, sleepers of various floor plans and observations. The Broadway Limited had a square end observation. Head end cars would be B60 style baggage and express and rpos. There were no domes operated in these trains. In later years, E8s could find themselves on secondary trains where there could be coaches either the lightweight streamlined cars of the P70s. By the early 1950s there were probably few cars remaining in the Fleet of Modernism scheme and it was unlikely that they would be assigned to the top trains since they did not match in appearance with the newer equipment.
So as for cars to use with your E 8 set, iy would depend if you had a specific train in mind. MTH has made at various times a 5 car 18 inch car set painted in the tuscan colors but it has a boattail observation . K Line has made similar sets and the K Line observation was a square end car. Golden Gate has done the Broadway in 21ich cars. They have also made P70s and heavyweight Pullmans in the 21 inch size. Weaver made the B60 style Baggage Mail cars in various configurations and with both round roof and clerestory roofs. MTH also had 18 in RPOs and baggage cars in heavy weight style.
If you were doing the South Wind, the prototype train was a joint venture of several railroads to offer Chicago to Florida service, so that train might contain aluminum fluted cars of other partner roads like Atlantic Coast Line as ell as Tuscan red streamliners I think the PRR took this train as far as Louisville and probably L&N took it from there.
I wouldsay for your E-8s, I would stay with a K Line or MTH Tusca set and add to it either the Weaver or MTH head end cars . MTH also had the RB50 reefer which could aslso work in your train as well.
If you are wanting to do a Congressional or Senator, I would favor a GG1. For myself, I am waiting on the Senator set and hoping to use it with a Williams Scale Silver abd Red GG1. Should be a nice mid 50s PRR Northeast corridor Train.