I like the idea of Ross switches in important areas and MTH or Lionel switches where derailments are not a big issue. Since my room is not a rectangle, I don't need exactly concentric track, but it would be desirable in some spots. I think the largest overhang would be from my 392e or my MTH/Ives 3245 (the big Olympian electric). The largest underhang would be the president's special AF cars which are the same size as the AF/Ives Olympian passenger cars. These probably need 7.5" spacing based on posts in this forum discussing SGMA spacing.
I will look into the tubular O track, thanks for the suggestion. On my last layout, I had one loop for O tinplate inside the SG loop. I used O-27 track with Marx O-27 curves that were about 34". It worked fine with all my tinplate, but I did not have any switches. Will tinplate engines such as the 263 work with O-27 switches - I'm guessing not. The 263 engine worked (just barely) on O-27 curves, but my Blue Streak train did not.