Hello, do you know which characters you're going to be trying to audition for in each show? Without knowing the specifics...
For Little Shop, I would look into Motown songs, other Alan Menken songs, and some big ballads if you're going for Audrey. Try to find something that shows off your voice that you think you could perform well. Maybe take a look at Hairspray and The Marvelous Wonderettes too.
For Sweeney Todd, I would look into other Sondheim shows, especially to prove you can do a patter song. The slower stuff is mostly for the guys. If you're a mezzo/alto, I'm guessing you're not going for Johanna. I think Into The Woods might be the best place to look. Possibly Company if you're creative. You can interpret a song differently to convey how you would play the character.
But there are no rules. You could do Adelaide's Lament and if you sell it and they like you, they'll call you back.