Flexibility is key. I have an oval and a branch line. Having recently got an HST I am considering making some temporary platform extensions. I can add these when running modern era. And remove for steam only running. Many stations have longer platforms now than when first built.
Running Willy Whitelaw, Falcon and Blink Bonny in steam. 08 shunter in blue, HST in blue/grey, Ed Elgar on order. Rolling stock is Pullman (starter set), mk1s in crimson and blood and custard, mk3s with HST. Mix of wagons.
Taking Alton (Hampsire) as an example I can mix heritage and main line, but of course to be truly authentic to modern era this would need EMU stock. You can't have everything. I suppose NYMR is another case of a joint modern and heritage steam layout that would also fit LNER locos.
I think a real dream operating rule is some way off as the ranges is still young. However I have given up on GWR and Southern for the time being!