Electrical panels, and use...

I have an attached garage addition, w/ upstairs living areas (all new construction), and would like opinions as to use of a particular breaker box to power the addition spaces. I have a Square D 200 amp box for the older part of the house, and would like to use an older Federal Pacific box to power the addition. Some 110v. electrical outlets, lights, ceiling fans for the upstairs, and 110v. electrical outlets and two 220v. outlets in the garage.
The FP box has the following specs: Cat.# lx116-24, Stab-Lok, 125 A. max., 120/240 V., 1 ph., 3 wire. It has 24 spaces, w/ a 100 amp main installed. Configuration of the breakers and uses will be different than shown in the pictures.
The Square D box specs are: HOMC30uc, Series SO1, 30 spaces. All spaces are currently in use. A 200 amp main is installed.
I would like to jump off of the Square D box w/ the FP box. Distance between the boxes is about 6 feet (just on the other side of an interior existing wall).
Anyone see a reason not to do so? There are no inspections required in the area where I live, as we are in the county. I will let the electrician obtain the permits if any are needed. Again, in the county, and may not be required.
Pics are enclosed, but are not the best quality, by a long shot. (New camera, and haven't got dialed in on it yet) Sorry for the quality.
Thanks, David.