Harry Kim is senior staff because of his position as the Ops officer, not his rank. He's a lowly ensign, the lowest officer rank.
This seems reasonable at the start of the show. He's the best person for the Ops job, so he gets it based on merit, and he's fresh from the academy, so he hasn't had time to get promoted. However, by the end of the show he's been a member of the senior staff for seven years, and he's still an ensign. This seems completely absurd. The in-universe argument presented by the show is that it's a small ship and there simply aren't enough command positions. But, as Ops officer, it seems to me that he already has a command position, and he should simply get the rank to match.
But, for the sake of argument, let's assume that that reasoning makes sense, and that there simply aren't enough people on Voyager to have one more lieutenant. This means that Voyager must already have the maximum number of lieutenants and above for the ship population. How many is that?
How many people on Voyager actually outrank Harry? Who are they?