I am a safari planner and it is super easy to combine Tanzania with Rwanda. There is a direct flight between Grumeti and Kigali on Grumeti Air/Coastal. I like to put gorilla trekking at the end of an itinerary, but it also works at the beginning. As far as time in Rwanda, definitely leave a bit of time to explore Kigali. Its a great city and the Genocide Memorial is something everyone should see. Then head to Volcanoes National Park by road or helicopter. I love both Kwitonda and Bisate (not as fond of Gorilla's Nest). A 3 night stay leaves plenty of time to gorilla trek, golden monkey trek, enjoy other activities and spend time at the lodge. Magashi is a great little camp which can thoughtfully be added to the right itinerary (as can O&O Nyungwe). Rwanda is a stunningly beautiful country and gorilla trekking is a bucket list trip that definitely lives up to the hype.