Ooooh, lucky you, Kass! The tan 500 is referred to as The Rocketeer, because of it's mid-century space age look. The slant shank machines have their own kind of feet, so I hope you got the accessories with the machine. They are fun to sew on because there's so much visibility! That slanted shank does make that difference.
And the 15 should run well too, once you clean and oil it. It will be great to have a treadle to sew on during prolonged power outages, should that happen. Or you could set it up out on your porch in nice weather and sew outside, like Bonnie Hunter does!
Have fun with your new toys!