187nj,
To answer you question you can mix and match flight schools, but you must have a copy of your student records and exam certificates to take to your new school, so don't worry about that.
I think you have a sensible attitude with regard to the exams. If you can have them all done, it will make you life alot easier with regard to your flying. I did my PPL on one of these 21 day packages, infact I think all the flying was done in just over 2 weeks
. Looking back and knowing what I know now about instructing, it was a little too much. I remember getting through the first few days and thinking I am not sure whether I can cope with this. This simply was the result of being so bloody tired! Anyway, as long as you put the work in and you have the weather, you will be fine and complete the course in the alotted time, but you will be knackered!
Enjoy your flying, it really is good fun.