Ross track distance between track on curves

On curves, you'll want to have at least 5.5" center rail to center rail spacing in your scenario, so O72 and O84 will work.  The 4.5" center to center spacing of the Ross crossover is fine, since it is on straight track.  Just let your track spacing increase when you get to the curves, and decrease when you come out of the curve.  Gargraves flex track is great for doing this.

4.5 is ok for the straights.  You may need 5.5 - 6 on the curves.

There  are many posts covering your question. Years ago I made wooden track spacer guages with different measurements.

The back swing of your big boys is where you have problems.

The MTH Premier Big Boy has the most overhang of what I run on my layout, it's 3.25" from the enter rail on an O84 curve.  I have an pole that's just over 3.4" from the centerline, and the BB misses it by less than 1/4".  Usually the inside of the curve isn't as critical, but there is one consist that I got burned by a 2.25" inside curve clearance, the new Lionel Rocket Booster Train.  The cars are 3.25 wide, and the clearance car is 3 7/16" wide.  That car clipped the 2.25" inside curve support and I had to make an adjustment.

If you put the 3.25" + the 2.25" clearances together, you get a 5.5" total clearance of those two trains happened to be passing on the adjacent rails.

Other than that, my 5.5" on curves has not been a problem with any of my other equipment, and I have a lot of articulated steam, the Centipede diesels, the C&O Coal Turbine, etc.