Hi Mick
A 10" Newt is not the best way to get into astrophotography if you've never done it before. Likewise you'll find there are much bigger issues to learn and sort out than what type/size of guidescope.
A 50mm guider and sensitive guide cam is quite capable of guiding that scope, however the scope itself is likely to have flex in the tube around the focuser that will be a much bigger issue than than the guidescope, depending what you will be imaging with and how heavy it is too of course.
If you're really fixed on getting a Newt, would suggest don't go over 8" or below F5 focal ratio. Seriously consider getting a 2nd hand ED80 instead.
A newt is however a good compromise for visual and AP starting out as long as you're prepared for significantly more challenges than a shorter length refractor. There are lots of helpful people on IIS that can answer your questions and point out the key issues through your decision making.
I think you're in "the rough" at the moment worrying about an 80mm guidescope however.
