The intuition is: all the debris from a collision now has an orbit passing through the point of collision (at that point of time). Collision at 300km means all the debris must have an orbit that passes through 300km at that moment.
The orbits will be elliptical: exactly how elliptical depends on the directions of the two objects hitting each other and the dispersion of the fragments (how much the fragments have sped up or slowed down compared with original objects, and change of direction).
The fragments the stay in orbit the longest should be those that remain closest to a circular orbit?
The orbits will be elliptical: exactly how elliptical depends on the directions of the two objects hitting each other and the dispersion of the fragments (how much the fragments have sped up or slowed down compared with original objects, and change of direction).
The fragments the stay in orbit the longest should be those that remain closest to a circular orbit?