Bloodmoon is fine to have right from the get-go. Tribunal is the problem, though. If you want to go pure vanilla (no mods) for Morrowind, either use Tribunal after you make decent headway into the game (level 20+ or something), or, like you said, start Tribunal after finishing the story.
If you are not against using minor modifications, you can have both expansions installed grom the get-go and use a mod like this: http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=2487
You won't miss out on any gameplay features by not using either expansion from the start, I believe (I recall some Menu/User Interface tweaks from using Tribunal, but it has been awhile). I personally would have both expansions going from the start, and use the simple mod above to make the DB assasins only attack you once you are well into the game.