My G scale shay has the same issue with tight curves. If the drive shaft is on the outside, the flexible sleeves slide right off the drive shaft. On the inside it'll bind up if too tight. I want to say I run it on my grandpa's 5' curves, but I'm not sure if that's the exact diameter. Could be 6' too and I usually put a straight section on each end of the oval.
Not sure if you can do this on your layout, but is it possible to put some straight sections between curves?
