For tightness, you can both bend your knees and take your feet wider. Those are both pretty common modifications for tightness in the low back and legs. To play with how your shoulders should feel in down-dog, do it standing with your palms flat on a hip-high countertop. My kitchen counter is the perfect height for this for me but ymmv. There you can play with taking the tops of your shoulders wider and expanding through your chest without encountering as much tightness in the body to inhibit you. Paul Grilley, mentioned by indywind below, has several resources that may be worth purchasing. I've not used his yin yoga video but I have done at least one workshop with him and I like what he teaches quite a lot.