I'm finding the bosses at the end to be very easy with Chopin, Jazz and March/Salsa. March and Salsa build up combos so easily and they hit surprisingly hard. I really find no purpose for a healer on my team. Sometimes I will sub out Chopin for Alegretto or Viola.
Also, Viola doesn't join the game later. She joins it pretty early. You might be thinking about Falsetto.