Grand Admiral Kim of the Terran Empire? Has a certain ring to it. He crosses over from the mirror universe having caught wind of his duplicate's failure.
Janeway had the authority to promote Starfleet officers under her command, but not the authority to recruit new officers. Hence the "provisional" part. But the situation required it so all the Maquis she integrated wore those alternate pips until Starfleet could ratify and approve them (most, if not all of which, they did.)
But didn't she recruit Tom Paris? Or did she just "restore" his rank of Lt? If she had the authority to restore a criminal's rank, surely she would have had the authority to recruit, either way.
If it makes you feel better, Captain Chakotay had normal pips by the time the Protostar was commissioned. Guess Janeway just didn't want to grant ranks away from Starfleet proper.
I just always imagined an awkward conversation between Janeway and some new species she had just beamed on board: “so uh, Captain, why do some of your crew have different rank pins?”
“Oh, you mean the terrorists? Don’t worry about it, they’re fine.”
I'm pretty sure either one of the recent episodes of Lower Decks, or the episode of TNG with that guy that doesn't look like Tom Paris, had a minor character that was some kind of junior captain, with three full, and one hollow pip