Whatever you use the curve pieces have to add up to 90 degrees. You can't, for example, use a curve that has 8 pieces to a circle (45 degrees/section) with a curve that is 12 pieces to a circle (30 degrees per section), unless you do some creative cutting.

I've heard of O72/O54/O72, but not down to O36, let alone O27.

Fastrack O36 is a 45 degree piece and the O72 are both 22.5 degrees so that combination would work. Also tried it in AnyRail just to make sure. My main question is can a longer locomotive navigate that type of spiral?

While I'm no expert, my guess is that longer engines would still not work on your planned spiral easement.  One of the factors that determines an engine's minimum curve is the amount of swing needed for the trucks, and O36 requires a lot more swing than O72.  There is even a good chance that going from O72 down to O54 might not work.  Remember, easements are meant to be used from straight to (I believe, minimum) curve to minimize physical stress on moving trains.  In models, they may look nice, but they will not allow an engine to easily travel below minimum designated curves.

Chuck

I don't think the that engine will work because it's rated for O72. Had it been rated for O42 or maybe O54, you might get away with it at low speed. Just so you can see how much difference the O36 makes, I did this mockup using FasTrack O36/O72 and O72.