Dear Andrewww,
Glad to hear of your interest in Yoga and the progress you have made to finding comfort and stability in your body. Yoga can provide you with tools to care for your body-home and keep it a happy place to live.
Without seeing you and your practice, it is difficult to advise you about specific modifications or awarenesses for your body type and practice habits. I would also recommend that you see a Structural Yoga or other qualified Yoga Instructor in your area for a personalized practice which can address your specific goals.
In general, men tend to have tighter hamstrings than ladies. Further, the low back and hamstring tension often go hand in hand. However, regular stretching applied to the belly NOT THE ENDS of the muscle (tendon and ligament) and done with loving kindness will encourage a greater resting length. Don’t overlook the condition of the opposing muscles Quadriceps, Psoas, Abdominals. They may be contributing to your feeling of overall stiffness.
I would recommend Surya Namaskar (Sun Salutations), preceded and followed by Cat lift/ Cat tuck. This practice is a great overall body opener. During your practice of Downward Facing Dog, try practicing this pose with knees bent generously - until the back is aligned in neutral. Press the heels to the floor ONLY as far as you can until you feel the low back reacting (rounding). You will definitely feel the stretch in the legs, but focus as well on the strength and stability of the core of your body (front and back) as it maintains a neutral spine. Your breath can help you to free the constriction of the legs. As you maintain the pose, the breath is your key to relaxation. Send long exhalations through the legs, encouraging a softening, but accepting the moment as it is without resistance.
As for the soreness in the tail bone, a postural evaluation would help here. There are several gluteal muscles which attach to the tailbone. Perhaps you are stressing these with “clenched” buttock muscles, or perhaps you have an old injury? Looking at an anatomy book may give you clues as to the muscles which connect at the point where you feel the soreness.
Namaste,
Chandra