Will jockey wheels from a 10spd M773 rear derailleur, work in my 9spd M750 one?

almost certainly yes but the ‘centron’ sideways movement feature of the upper guide wheel has been more or less removed in 10 and 11 speed. It was a feature of 9 speed shifting that allowed the chain to centre itself on the sprocket after a shift but if your gears are well adjusted it may still be fine.