Can I use salted butter for this?

Yes, if you use salted butter, omit the salt step, which is missing from the recipe anyway. If you use unsalted butter next time, yes, add it in with the milk, water and butter.

I might add one thing to this recipe. Instead of taking the pate a choux out of the oven when done, I’d turn the oven off and prop the door with a wooden spoon or spatula for 10-20 minutes. This will prevent the steam inside the pate a choux from deflating all your hard work. Good luck!