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Guiding with your 150 should be np if you have a reasonable mount and guide camera. If you are thinking of doing the guiding manually, then you will need around 1.5x your imaging focal length to start with. The longer (within reaon) the better for manual guiding.
Flexure can be a problem for longer Focal lengths but shouldn't be a major issue with the 120, especially if you use a Focal Reducer and Field Flattener and the imaging scope and a good set of guiderings on your guidescope. I like a SbS mount for the flexibility it gives, plus it keeps the weight closer to the mount, but you will still need some way of pointing your guidescope independently of the imaging scope if your guide camera can't find a star to guide on in the field of view. Longer exposure for you guide camera can help to get around this, or having a wider field of view (consider a Focal Reducer for you guidescope maybe).
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