[Gama]

Having a good guidescope is OK.
I use a vixen VMC110L on my 20" CDK, and it has a longer focal length than yours, and i get good round stars, with no tracking errors.
I cant go lower in focal length on the guider, as i will start to get errors.

I am using the Planewave mounting plate for the guider, and there is no flexing that i can see in any images. The Vixen guide scope is very light, and small, and is a perfect match.
I have attached an 11 minute exposure of IC-3639 during my Nova hunting sessions, and stars are nice and round.
For what its worth, i have the guider mounted on the bottom of my scope, and that i have a Fork mounted telescope. The mount is a MI750, and i suppose there's much less flexing on the scope as apposed to a German mount.

Theo