I sorta hate to say this because it will sound argumentative, but ...... the hole in the floor with a plate in a conventional fireplace is typically an ash cleanout trackway in my experience. The exterior door typically mounted on the chimney may not be there or may never have been mounted, thus making it appear to be an air feed. In your case, maybe; maybe not. Either way, my recommendation would be to remove the door/plate from the bottom of the fireplace, insert ceramic fiber insulation in it for closure.
If you are referring to the Jotul C550 Rockwell cast iron insert, I can see a couple "challenges" that may surface during an install into your 24"/25" fireplace. 1) Once the insert is set under the best conditions there will only be 1 1/4" space for someone to get their arm into for the purpose of connecting the flue pipe. The CC of the connection is 12" back which would prevent me from making the connection. (Not saying it is impossible, it would just be a challenge)
2) The pictures do not indicate to me that the 1" stepdown extends to the sides of the fireplace opening. If this is true, the insert will most likely need to be raised 1" to allow installation of the fan, the bottom surround trim, and the side surround pieces. The bottom surround trim fits flush to the hearth and "trims" out the fan assembly. Naturally all the afore mentioned items interlock with each other, thus if one is out of allignment none will fit properly. If you are not referring to the cast iron model then the surround kit is optional. Needless to say, if the insert is raised 1" to meet surround mounting requirements then the top space is then only 1/4" and I don't think I can even get my finger in there to make the flue connection!
I just finished an insert install a couple weeks ago and these are just a couple possible "challenges" that come to mind. They may be worth exactly what has been paid for them, but I wish you the best.