Make sure you have read the v4.1 FAQ for the Dark Angels, it should answer some of your questions:
http://us.games-workshop.com/downloa...s_faq_v4-1.pdf
The major point being, the Dark Angels use the Command Squad from Codex:Space Marines now, not the one out of their codex.
Sergeants are part of the squad and cannot be replaced, but you can still attach other characters to the squad.