Use root to create new user, then run app as new user.

Or you could use a separate key on each device.

Anything Cisco that says FE is 10/100mbit.

Flatpacks include the dependencies with the application. So different flatpacks may have the same libraries over and over, wasting space. RPM/DEB install just the application and each dependency is a separate package, and packages that use the same dependency will share the one copy. So flatpack is better for consistency when running the app because everyone is running the same dependency version, and space isn’t as much of an issue anymore with nearly everything having more than enough storage.