That actually sounds very plausible. Lady Gaga seems exactly right, persona-wise, for Elphaba. And Taylor Swift has a prissy quality that seems right for Galinda, although she also a gangly quality that doesn't quite fit the role.
Both might be too old (Lady Gaga is 32; Swift is 29), or at least, Gaga may seem older than Swift rather than a contemporary. Because Swift has been in the public eye since she was 16, it's easy to think of her as a student. Gaga was only 22 when her first album came out, but she seemed like a full adult from the get-go.