While a .38 Super magazine may handle 9mm, I have found that to be unreliable. .38 Super cartridges cannot be used in 9mm magazines. There should be no need to "fit" a .38 Super barrel into a 9mm slide. Should be a simple drop-in replacement. I have no problem firing 9x23 Win loads in mine, but I normally use lighter loads. Starline .38 Super Comp cases work very well in my gun, but I also reload ordinary .38 Super semi-rim cases. If you want, for some reason, to load very hot loads for use in a .38 barrel with an unsupported chamber, using 9x23 Win cases is strongly advised. Except they are very difficult to come by today.

Regarding a .38 Super barrel, it should be one which headspaces on the case mouth. Old barrels headspaced on the case semi-rim, and you do not want that.

I have a similar setup with an EAA Witness. One frame, two slides, three barrels, and appropriate magazines and springs. .45 ACP, .38 Super/9x23, and 9mm all on one frame.