I built my Falkreath home and chose Lydia as a steward. Can I somehow get Rayya as a follower?

Rayya is a housecarl, not a Steward. All you need to get Rayya is enter the Falkreath longhouse, where you bought the deed for Falkreath's land. She should be standing there, if you did not ask her to follow you. Alternatively, she might be patrolling your home in Falkreath, as she is designated to live there, once it is built.

You need to be Thane of Falkreath too, of course. You can have both a housecarl and steward separately at your house, or you can choose to make them one in the same.

Can I still pee in my bucket if I equip the Boots of Befuddling Two-Step?