Whoah, been reading some more about this:
http://www.wilmslowastro.com/tips/autoguiding.htm
Talking about ratios of focal length of imaging scope versus guide scope, pixels of imaging camera versus pixels of guide camera, sub-pixel capabilities of the guiding software, arcseconds (what?)... getting compli-ma-cated...
Summarising my equipment considering the above -
Imaging gear: ED80 600mm f/7.5 with Canon 30D 3504 x 2336 px image size.
Guiding gear: ED100 900mm f/9 with SPC900NC 640 x 460 pixels.
Proposed guide scope: 80mm f/5 400mm FL with above SPC900NC.
I have no idea how or what to calculate arcseconds.