I run, build, paint and generally "mess with" only 3RO. On the odd shelf (or in a box) here and there I have a few pieces of 2-rail O, 1 MTH 1:32 NYC J3a Hudson, AF S Hudson and PRR K5 Pacific, an N piece, one TT, one OO Lionel Hudson, 2 or 3 pieces of HO, 2 or 3 pieces of Standard Gauge, a piece or 3 of On30.
But - 3RO is 95% of what I have and all that I use. I have never liked mixing scales on a layout (or in a room). Even for forced perspective, which is seldom successful, anyway. Mixing looks funny to me, and having 2 layouts (for example) in 2 scales...can't see that either. No focus.
Even On30 does not go with 3RO (it would with 2-rail O, in appropriate narrow-gauge scenes/interactions)
Why? The 2-rail On30 track just really calls attention to the middle rail in 3RO - and not in a good way. With nothing for comparison, I can almost forget it. Almost.