Hi, looks to be an interesting project you have, You may want to view the threads on Torrington & Treneglos on here as they are layouts that depict steam running on the Southern 'Withered Arm' which is what sounds like you want to do. In view of what may have run into those stations, T9, M7, West Country's from the Southern, and from the GWR Praries, Panniers, Diesel Railcars, Moguls, and possibly using Manors and counties for the bigger passenger trains. Will continue to watch your progress on this.
Colin