The ones needing the larger curves are the scale ones. The locos that are packaged in the original conventional set and then the visually identical LC set will run on O27, I've done it. I acquired some Marx O34 couple of weeks back, 2 complete circles. I am playing around trying to figure out my Christmas layout and I had the conventional PE Berkshire running on the Marx O34. No problems at all.
The Berks are actually a very old design, originally designed to run on O27. I think the middle 4 drivers are flangeless to let it make the turns without jumping off the track. It looks a bit awkward but it did do it. I just put the turn arounds out of sight so you just saw it going one way, then back.
Frank