is the AMD A6-6400K compatible with the AMD radeon HD 5450?

It is, but there's little point. The HD5450 is little faster than the integrated GPU, and hybrid crossfire has terrible frame consistency, often leading to even choppier gameplay than without it enabled, even though in theory you're getting more frames per second.
 
Even with crossfire with APUs, I believe there's only a small subset of recommended graphics cards recommended for the task. And a good graphics card will give you more than what the crossfire can achieve.