My layout allows operation of equipment with either scale or highrail wheels. I do it routinely. The track is MTH .138 rail flextrack. The turnouts were custom built on FasTrack jigs using the .138 rail, the jigs required modification to accommodate the .138 rail. Some design features of the layout are minimum #5 turnout, minimum #6 mainline turnout, all mainline through routes use the straight leg of the turnouts and the minimum radius is 30" with easements. If anyone wants to see videos of similar turnouts with both scale and highrail equipment Tom Stoltz still has them posted on his website.
There are some unforgiving items when doing this. For example all highrail cars need the wheels perfectly gauged or they will jump at the turnout guide and wing rails. I had to regauge about 80% of my Gilbert cars and 100% of the Lionel production cars. All the MTH and American Models cars worked right out of the box.
Just because scale wheel sets work on the layout does not mean S scale trains can be operated on the layout. With a 30" radius only scale diesels and small steam engines will track through the curves. Most S scale steam engines seem to need at least a 36" minimum radius. Also at a 30" radius cars with body mounted couplers need to be coupled to similar sized cars, an 80' passenger car will not work coupled to a 40' reefer. The main challenge with visitors scale equipment is all the S scale operators I know use either DC or DCC. My layout uses Legacy and conventional AC, so the visitors scale engines cannot be used.
As others have said, no problem making transition cars for the couplers.