Hi Abi, Sikkim and Meghalaya are good to go for october. I'm attaching with this comment few packages that you can use these packages to go ahead with your plan. You can click them to find more details about them and use the same link to book these
Sikkim During in winter - From October till March
Winter is the best season to visit Sikkim, if you love snow. Snowfall at Yumthang valley, Gangtok, Pelling and Lachung make it appear heavenly. Temperature in this season stays between -5 degrees Celsius to 7 degrees. Meghalaya experiences autumn between October and November and winter season from December to February. The average day-time temperature is extremely pleasant at 15°C, but the night-time temperature drops to around 3°C during the winters. Foggy days are typical here. It is a treat for those who wish to get engulfed in the Fog amidst beautiful surroundings left behind by the monsoons. One can unleash the explorer in them and go sightseeing or participate in a trekking expedition to traverse through the thrilling mountainous terrain. Shillong, Cherapunji, Tura, Nongstoin, Nongpoh, Guwahati, East Garo, Jowai etc. are visited often during this season to explore attractions like Balpakram National Park, Umiam Lake, Laitlum Canyon, Double Decker Living Route Bridge, Ialong Park etc.