I have absolutely no experience with hemlock (Tsuga) species but most conifers can be trained as bonsai so why not try them out? In general it would be a good idea to wait until they are growing vigorously before doing any serious pruning. They should respond to pinching by producing tighter needle structure and shorter internodes. You might inquire with bonsai clubs in your area to find out if anyone is using them. I would be willing to bet that someone is. They won't survive in my area so I have no experience with them. Good luck Jack