Funny you mention bed sizes. We upgraded to a king so that there was a bit more room and DS has our queen bed in his room. It does work, I'm just too lazy to get up in the night anymore lol.
He has a safety rail on our bed but not on his own as he sleeps in the middle IYKWIM as opposed to on one side.

DD never had a rail and we put pillows down for the first while, but she never fell out at all.