Hi John.
Mounting will definitely be a problem. You don't want any flex, and I can't see eliminating that with any mount to the truss tubes, short of actually bolting/clamping something on with three points of contact. Have you considered an off axis guider?
I have the Apex 90 and picked it up for playing with guiding, but have yet to really try anything serious with it (I had 2 kids instead). It does seem to be fairly solid, and I have not noted any significant mirror slop, but guiding with it will definitely put it to the test.
I'll let others address the focal length question, as my expertise does not yet lie in that area
I was going to spout something about a 2:1 guide to imager focal ratio, but it would seem that modern guiding software can overcome that limitation.
Eric