At the end of May, I would DEFINITELY pre-purchse tickets to the Uffizi online. Queues there will be very long.
You can do that here:
https://www.polomuseale.firenze.it/en/musei/?m=uffizi
We did the Pitti museums at the end of May a couple of years ago and practically walked right in; almost no queue at all. I don't know if things have changed but I would guess pre-purchasing tickets isn't necessary for that one. The Palatine Gallery - the best of the Pitti collection - did get pretty crowded in the afternoon, though.
Find all the information here:
https://www.polomuseale.firenze.it/en/musei/pitti.php?m=palazzopitti
Yes, there are varying ticket prices for different of the galleries or the gardens so it's easiest to just buy the combined ticket to the whole complex unless you know there's only one museum you wish to see. Click on any of the museums in the link above, and then click on "combined tickets": Translated from Italian:
"Combined ticket for the entire museum complex of Pitti Palace:
€ 11.50 (valid for 3 days - full ticket)
€ 5.75 (valid for 3 days - reduced ticket) To assess the cost of the ticket the visitor is requested to check the opening days and times of each museum"