My layout is all 027 with 027 curves. Stick with smaller diesels and steamers. Geeps, and end-cab switchers work perfectly. I do have a few engines rated for 031 curves which do work on 027 but some of the low body details like fuel tanks do hit the covers on 5121 turnouts. Not as drastic as Curt's solution, I shaved off the corners on the leading edge of the covers and solved the problem.
Steamers present different problems with cow catchers and cab hoods.
In my next life I'll build my dream layout with the endless resources that I will have in fantasy land. 
