If you plan to combine gorilla trekking with chimpanzee tracking, then January is not the best time. Chimpanzee tracking in Nyungwe is actually better in the wetter seasons (mid-February to June, and mid-September to mid-December). This is due to them being easier to locate as the fruits of the forests ripen, so they tend to munch away closer to the forest floor in larger groups.
You can combine Rwanda with most Africa beach destinations in January, from Zanzibar, to the coast of Kenya. So if you are looking for a wildlife and beach combination, it can be a great month to travel.