According to the 3.0 ruling, any creature with 1 racial HD drops that HD when gaining class levels. 3.5, though, has a new ruling:
So, with the 3.5 ruling, a Pixie Sorcerer 1 would be a minimum 6th level character (1 HD Pixie, 1 HD Sorcerer, +4 level adjustment). But what about a Jermlaine...