don()shinobi, normally in FE games, everyone has 1 class tree that they stick with, and promote to higher levels within the class tree. In Awakening, each character has several class trees (usually 3) that they can switch between with the use of an item. When reaching a certain level in a class, they learn a skill, which they can have with them anytime even if they change classes around.
What the guy was saying is the beast rabbit girl thing can take on the Dragon Knight class tree and is really good at it. I have not confirmed this for myself, but it is something that's possible.
I can get you my recommendations on who should marry who; I posted them on a profile wall and I'll have to go dig them up.