Can you use a higher level pearl of power to regain a lower spell slot? Say you have a Pearl of Power 3, could I use it to regain a 1st level spell slot, or is it specifically only for that slot and no others?
Strictly speaking, it must be of that level:
The spell must be of a particular level, depending on the pearl.
The D&D 3.5 FAQ agrees:
Can a pearl of power be used to recall a spell of a lower level than it's designed for? After all, I could have prepared a lower-level spell in that spell slot.
No. A 5th-level pearl of power can recall only a 5th-level spell, even if all you prepared as 5th-level spells were multiple castings of stoneskin.
Any metamagic effects (or other effects that altered the spell's level) apply, so a 5th-level pearl of power can recall the quickened magic missile you've already cast, but not the empowered cone of cold (since that's a 7th-level spell).