I think it will depend on the ticket clerk you get. As I understand it, an overdistance excess (which is what you'd need) is only available for travel to stations beyond your original destination - which Paignton isn't (only just). If the ticket were from Newton Abbott, it would be straightforward.
I'd be inclined to try a few people/places if you don't have any luck first time round. If you get a train direct to Paignton from Salisbury then I reckon there's a reasonable chance the guard would accept it (if you find him/her first and explain) without the bother of excessing it - but if not, you'd be buying a full-fare single Newton Abbott - Paignton.
(Rather ridiculously, if you had planned ahead, a way around this would be to buy initially from Newton Abbott and excess the outward to go from Totnes, and the Return to Paignton - perfectly valid and costs the same to you (£0.00), but twice as much work for the railway!)