No worries.
Seriously though, I've been a 'regular' at that place for a long time now -
I'll runout to Tongue and then cut back home via Altnaharra or go via Duress/Ullapool/Lairg and have never had any problems at all. The guy always speaks and I've never heard anyone mention him running out of fuel...maybe I've just been lucky.
And the shop has always been reasonably well stocked. You have to bear in mind that it's a small shop on the edge of a small village in a rural community - and there's another shop in the village centre...hardly going to pack it to the gunwhales with unsellable stock, are you?
Regarding what the numpty in Bettyhill told you - I'd take anything they say with a pinch of salt. They can't piss straight let alone give you a straight answer
Back to the real issue:
RCE, bear in mind it's 140 miles from Thurso to Ullapool round the North West route, so with a bit of jiggery pokery with a can or one fuel stop you'll be fine.