We will be driving to Disney this year rather than flying and may have a partial day at the MK the first day or we may not. Our plan was to buy a 6 day ticket even though we are staying 7 nights and add a day later at guest services if we need to.
At our 60 day window are we allowed to book FP+ for every night with a hotel reservation or would we be limited to the number of days on our ticket?
You will be limited to the amount of days you have a ticket for. If you add a day you can add FPs at that time for the extra day.
Edited to add- if you have a discounted ticket and originally you do not have FPs for arrival day but want to add them you will need to enter the park (lock in the price), leave and go to GS to add the day and then make your FPs . If you make FPs for that day without doing this it will cancel one of your other days (since you will not have a valid ticket for your 7th day at that point). If you bought tickets direct from Disney you can add the day before you enter but in that case I would just add the day now and make FPs for 7 days since it is only about $10 a ticket to add the day.