Can I have doors of different heights within the same house?
For example, Our ceiling height will be 9' throughout, but we will drop the height to 8' in the bathroom, pantry, and linen closet/bathroom hall. I would like to put 80" doors on the rooms with the lower ceiling. On the entry to the mudroom, Dining room and other exterior doors I'm thinking I should go with a taller door - maybe 90" or 96" in order to keep in feeling proportioned to the ceiling height. What do you think?

My front door is a round top 96", 4 hinge fiberglass and the rest are 80". 80" looks fine with 9' ceilings. You will be shocked with pricing when you get over 80".

Yes, different height doors works well. Just use your own sense of aesthetics, consulting with others you trust (or who might be living there with you.)