Height Ceiling of combat aircraft

Quite a simple topic today, What is the ceiling combat aircraft can and do fly at.
This came about from reading a article about the X-50 scramjet plane thing. it says it will be taken up to 50,000 feet by a B-52.
Can i B-52 fly this high? Can it fly higher?
What are the resrictions on aircraft flying even higher? When you look at most combat jets they seem to fly about medium altitide 15-20,000 feet. I know passenger jets often fly 38,000 feet.
Here is a basic list of combat aircraft from around the world. Is there any big variations on what they can fly at and what they normally fly at?
Tornado GR4
Tornado F3 ADV
F16
F15
F18
F4 phantom
Mig21
Mig 23
Mig29
Su-27,30,34,35
Su-25
A-4 skyhawk
E-3 sentry
E-2 hawkeye