That’s 750 too many. Are they even authorized to be there?
I don’t understand the question. When career people are promoted high enough so that they are the boss of GS-15s, they generally go into SES.
Another characteristic of the SES is that they report to the acknowledged political appointees.
So someone like the FBI Director is frequently from outside the agency while the Deputy Director is career and SES.
Sometimes federal law designates those second-in-command people be career. (Not sure about the FBI specifically).