I have a 2 story house, built in 1991. One of the downstairs rooms has an 8' ceiling with a large space above it that I would like to convert into a room. I don't want to use the existing ceiling, and would like to install dedicated floor joists.

Here is a description of the existing layout.
The second floor has a smaller footprint than the first.
The attic area above the room is arched and has a common wall with the stairs going up to the 2nd floor, a vaulted room to the back side, an outside wall opposite the stairwell wall (those 3 walls thus being framed...) and a smaller wall above the neighboring room on the front side, (which shares a common height ceiling with the one in question).

So, I have a 10' x 11' room, with 3 framed walls, and a partial. I have access to it through a future door at the top of the stairs. This space is vaulted, a peaked roof of the 1st floor abutting the smaller footprint 2nd floor.

After I cut in the doorway, I want to add new joists, above the light ceiling framing that currently exists, so that no load is added to that ceiling. I'm not sure how the wall framing of the 1st floor transitions to the 2nd.

Is there a standard framing format from the 1991 era to which I can add hangers?

Do I need to add ledgers, and if so, how do I add them to the INSIDE of an attic space not intended to be utilized?

Being intended as a storage room, should I go with a 16" joist spacing? The room is 10' wide. Do I use 2 x 10s at 16" for heavy loads?