Nitpick: Paris was not Maquis. He was a former criminal given a reprieve. Not sure if he had a rank when Voyager sailed in S1E1 of the series but I think he did. He was part of the crew and driving the ship.
More broadly I can see captain Janeway on Voyager needing to give field commissions to people on the ship. They were many years from home and the ship needs to function. Maintaining a command structure is important so you start slotting people into various positions and give them ranks so everything runs as it should.
Whether Star Fleet agrees once they return is a different question (if they never went through Star Fleet academy do they get to keep the rank? Do they get all their back-pay at that ranks for all those years? Do they get pensions commensurate with time at that rank? What about people who never got promoted as they normally would have?). Star Trek hints that there is no money in the future but they are a little loosey-goosey about that.