You *can* make a dithered intermediate radius by alternating 18" and 22" sections, which will give you an effective radius something like 20". Atlas has used this in their planbooks. Example: Their Great Eastern Trunk plan.
Lots of standard gauge equipment will work fine on 18" and snap-switches or no 4's. Stick to the four-axle diesels, freight cars 65' or less (perhaps 72' with care) and shorty passenger cars. Some 6-axle diesels do work. As you don't go in for the dismal-electrics, my roster which consists of a Bowser H9 and smaller steam works fine on my 18" curves, as probably would most mass-market Mikados and Pacifics.
One recent layout in MR had 12" radius HO curves. The owner used small steam switchers and short cars. It was something. I'd have to see it run, however.
It /will/ be an interesting railroad adventure. You have also picked a prototype and era I am growing to like very much - the classic PRR before the war, diesels, and duplexes. If you have any questions, feel free to PM me; I might have bummed the answer off somebody already. Bowser has lots of metal PRR locos and plastic PRR freight cars and cabooses, so equipment is easier than for many railroads.
Do try to find out how people make their track plans. You won't necessarily want the independent loops forever, but you can always rearrange and reconfigure.
