YES, you can trim in flower. You can prune lower branches that are spindly and will not produce anything worthwhile. But I would wait until they are a little further along, as this is the time that the hormones are going through a large change. It's best to just leave them be until at least 4 weeks in, IMO. As long as you trim off the whole branch nothing will grow in it's place and the wasted energy will be put to better use. You can also take off dead fan leaves later on, they are useless as the leaves stomata is no longer collecting lumens for energy and transference is no longer taking place. Does that answer your question?