Which of these stretches can be harmful? head circles tricep stretch quadricep stretch achilles tendon stretch?

The most harmful will be the head circles as this can put your cervical spine to undue tension and compress, and even permanently damage your spinal cord. The other stretches do not pose risks (i.e. a tricep stretch will not cause elbow dislocation) other than mild discomfort if done incorrectly. 

The organism dies boomshakalaka