So, sounds like your question is two fold. Yes, I think you can us believable buff in all the rooms, but you might want to check and see what the builder charges for an additional color. Some times it can be reasonable enough that it makes sense to go with at least one more color. As far as using BB for the ceiling too, I would be worried that it might feel like you are wrapped in craft paper! You might try the lightest color in the card which is Antique White 6119, but I would probably go for their ceiling white and spend the extra money on another color. If you want to play it safe just run down the card to a darker version of Believable Buff. Here are a few photos of the darker shades we used in another cottage. They run the expanse all the way down to Craft Paper.
Good Luck and send photos!