I know in past elder scrolls games some mods stopped working or caused problems until updated to the most recent version and it was always the safest to uninstall them until they had been worked out. Is it still the same or can I just keep them installed?

I don't know if Dawnguard constitutes a new version of the game. It's just add-on material.

I doubt it. Only if any mod is unfortunate enough to edit cells where Dawnguard puts stuff in.

I'd do it just in case, there are new areas.

Now THAT I did wonder about. That would svck to be that modder, I admit.

Well I would hazard a guess that its got new animations in it for crossbow usage and vamp lord movement, so any mods that add custom animation will probably break.