Hi,
I don't have very long focallength, 910 mm refractor as main telescope. To that I use an telephoto lens with 400 mm fl as guide telescope. The main camera is a Canon 6D fullframe. The guide camera is a qhy5, just a bit smaller pixels compare to main camera. Works well and I have not noted any flex between the tubes.
See here how it's setup:
http://www.astrofriend.eu/astronomy/...flattener.html
Earlier I had off-axis guiding, if get in trouble with the new setup I build a new off-axis system to the new 3" field flattener. But it will cost a lot of money so good if I don't have to.
One advantage with a separate guide telescope is that I can have a wider field and then easier to find guide stars.
Lars