You can easily set default units in Photoshop preferences, but they doesn’t seem to have any effect on free transform (⌘T on the Mac), which always start with percentage as unit.
I’m aware that you can change units with a right click on the text, but it will be back to percent when you restart Photoshop.
Is it possible to change this behaviour? Can I have it automatically set to pixel from the start?
I’m afraid not, I’ve scanned the Adobe Forums AND the preferences of Photoshop itself some time ago because it’s (quite rightly) irritating. I think it is just as the software works. I know you can change the units in the preference menu, but it doesn’t seem to affect this.