You can use tongue and groove or shiplap. I would not use a simple butt joint, as it will leave you with gaps - no wood is perfect and even if it were, as it expands and contracts, it would create gaps.
Use a soft finish. Hard finishes like the water-based poly will fail if the wood underneath gets dents.
I would consider laying 1" pine over a plywood subfloor, using a layer of rosin paper to keep down the squeeks. You don't have to, but 1" softwood is a little thin for one layer.