If the plants are not going to be planted in the ground you do not need to trim the roots. If you want to you can remove about 2" around the sides and off the bottom and plant in any good potting soil. You can do the Pine any time from now until March if you root-prune or any time at all if you don't. I would wait to do any pruning, branch or root, until late winter or early spring.
One thing to note is that you will have to break the pot the Stewartia is in to get it out and do the same if you ever want to transplant the Umbrella Pine from that new pot. Both are narrower at the top than in the middle.
