Precisely, totally, exactly what I will be trying to recreate. Same period.
In your scenario, if you even push it out to 1964/5, you can include an occasional brand new Craven or 141, and obviously even in 1963, black'n'tan carriages are just starting to appear.
In your scenario, old GSWR stock, Bredins, laminates of various types and Park Royals will be interspersed with much (if not most) of a myriad of old GNR types, variously in brown, navy & cream, CIE green in black'n'tan, while the odd laminate is still very dirty "silver".
You've old GNR parcel vans in brown or green, and brand new CIE "tin vans", and even an occasional old GSWR 6-wheeled full brake all possible.
My own scenario is based somewhere in the "Deep South" - maybe Waterford, Co. Cork or Kerry, so ex-GNR stock while possible is less likely. About 1959/60 an old wooden brown GNR coach appeared on the West Cork system briefly. I might eventually have a green or black'n'tan K15......