All tinplate O-gauge will run on O-72 curves. If you have the room, I highly recommend going with at least O-72. As for switches, that's a can of worms for sure. I'm building a layout much like your description (outside loop of Standard-Gauge and 2 loops of O-Gauge) and opted for the O-Line Reproductions switches with 12 of them on my layout (formerly RMT, formerly K-Line SuperSnap). Like any switches, they have their quirks, but so far everything--from 1920s tinplate up to modern--runs through them and they've proven to be reliable. And an added bonus--they take up a lot less room than the traditional Lionel switches. I did a LOT of research on the different switch brands before going with these. If you don't have the room for O-72, that will limit what you can run, but not by much. Off the top of my head, the prewar trains that need O72 curves are: 763E Hudsons, Hiawatha, and M10000 City of Portland. The 700E Hudson / Rail Chief cars also need O-72 but also require T-Rail track. There may be more, but that's a good start.
As for Standard Gauge, all prewar trains will run on the old 42" curves, but now there are a lot more options of other diameter curves. As usual, the wider, the better if you have the space. I suggest contacting Kirk at USA Track, www.standardgaugetrack.com . His track and service are second to none. For switches, nobody beats the Standard Gauge switches made by Ross Custom Switches: smooth operation, reversible switch machine, and all-around solid construction.
Hope this helps.
John
