The .45/.400 combination you mention is a naural and I have had one for some years. Barrel change only. I built it on an old 9mm slide I enlarged the breech face on, so the next step was obvious - another slide in 10mm size, this one fitted with .40, .357SIG, and 9mm barrels. Yes, the same breech face works fine with 9mm (or9x23, .38Super, etc., as the difference is only a few thousandths between a .38S and a 10mm. You lose about 6 ounces of extractor tension, but function is fine. You can use a spare 9/10mm slide on a .45 bottom end by either changing ejectors(they are different), by modifying an ejector to fit both sizes of slide - or by machining the slide(s) to fit the other style of ejector. If you machine the frame for one of the ramped barrel profiles, and then of course have all the barrels in that profile, including the .45 etc, it makes for a lot more versatility. Some really neat calibers for the .45 slide are the new Triton .40Super and .450SMC, not to mention the .38 Casull. Count on switching recoil springs to around the 28# range for these. Awesome ballistics - check out the Triton and Casull websites! By the way, although you're not supposed to, you can reliably shoot .40S&W in most 10mm barrels, as well as 9mm in most .38Super barrels. I haven't tried 9mm in a 9x23 barrel - but I'm confident it would work fine, as the case is held pretty close to the breech face by the extractor.
I'd say as a safety measure after shooting a lot of "short" lead bullet loads in a longer chamber, CLEAN THE CHAMBER WELL before resuming, say, hot 10mm loads.
One of my favourite guns of this type was a stainless Commander-length slide/Officer's size frame (Caspian) I built to shoot 10mm, .357SIG HOT handloads (125's @ 1430 out of that short barrel), as well as 9mm, using the same slide etc. with three different barrels. Good fixed sights, coned barrels, ambi, beavertail, very nice wood. Brought it to a match for my friends to try, especially the "flame show" .357's, and one of them bought it! BTW, this gun used all non-ramped barrels.
Hey, the sky's the limit with a 1911!