Is there any reason why you shouldn't?
They will have a shorter life than decking as they won't have been pressure impregnated. But a very good idea, though. Treat it with Cuprinol (if you have a local, friendly timber yard they might have a pressure impregnator and then you're laughing.)
No reason other than scaffolding planks are way thicker than floorboards would be, thats all. If you've got a supply of them, you may as well use them, but they will be harder to fit. if they're going on top of joists, then the finished height will be above door clearances, too
I think that depends on the application and how you want your floor to look. They tend to be about 2 thick and wide (which is good for a scaffold). But most flooring is only 3/4 thick. If you have a way to resaw them that might be good. When I was a kid (1965) I worked in a factory with a wood floor and the boards were 2 thick and about 8 wide. This was a good floor when the factory was built about 1920. Not so good these days. In the right setting they could be very interesting and decorative especially if you can leave the old paint and marks on them.
