The consensus seems to be 22-inch radius minimum for reliable running for HO 6-axle locos.
My personal experience: I've got some 6 axle locos (Athearn SD40-2s, FP45s, and an Atlas C40-8) and 18-inch curves, and they do tend to struggle a bit getting through them. The longer 6-axle units also tend to exert a lot of lateral force through the couplers, which might cause either the cars behind it to derail or uncouple.
I started out wanting to run long trains with uber-power 6-axle locos like SD70MACs and AC6000s. But over the years I've grown to prefer 4-axle locos instead-- Geeps and switchers just look a lot more at home on a small layout.