Ah. Shooting natural eh. I just learned slow sync a week ago or so. Flashing your subject to expose them properly then making the background fill in for itself. The settings I used for a portrait was 1/30th at f/16 at 100iso. Flashes the subject and lets teh background come in. May be some blur in the background but depends on what your subject is of course.
Stopping the action on other hands, depending on relative distance, would have to be experimented with. I know a classmate of mine shot a close up of band members using slow sync and caught the singers still with huge color differences and diffractions and whatnot due to the slow shutter speed.
Its interesting. I"ll shoot you some links with some good images and try to analyze them. I can see slow sync being used for more closer stuff than farther. Now of course action unless you have lights setup and shooting far away, might not be as useful but you have lights. Once you know, experiment.