Mixing 0 and 027 stuff is an issue that's been around since before Hitler became a smoke unit. Plenty of ideas abound but here's my two dimes' worth (inflation). Having needed the room and donated my large Gargraves layout to a museum, the only space left at home was my workshop floor. No scenery except for a piece of workbench backdrop and a couple tunnels, plus some old Plasticville stuff at the foot of Mt. Power Saw. However, the are lots of operational possibilities. Without structures you can cram a lot of action in a smaller space. Footing a little tricky but I use a quad cane anyway so no big deal.
The mainline is 0, and with the exception of the long passenger terminal track, the sidings are 027, matching the uncoupler tracks. Rails are painted, but only the ones visible from the control center. Additional 0 ties are standard size, and my 027 ties are 3/8 x 1/8 x 2. Of course 027 rails are lower, but this provides a natural "hump" when handling cars. Small shims under the track ties support a smooth grading. Ties are Gorilla glued to the rails.
I have three operational trains and a yard switcher. The diesel passenger train, plus 0 and 027 freight trains. All the 0 train sidings are on one side of the layout, and 027 traffic on the other. I wish I had only 711 switches, but had to settle for a few latter day Lionel, and 022 switches on one yard due to space limitations. The painted rails, extra ties, and some ballast on the switches help the look.