You should go for both shillong and Cherapunjee..Both have different enlightened beauty to watch for!!
Sikkim(Gangtok) will be difficult to cover as it is far away!!
I too recommend to visit Shillong cherrapaunjee instead of sikkim
if possible go for one day home stay to get bathed in the beauty of nature completely.
Since u would be in guwahati, and not visiting an adobe of clouds "shillong". it's a crime. lol 😂😂😂... u must visit, it's a four hours of drive u can take share taxi in RS 400. visit cherrapunji, dawki, Bangladesh border, living root bridge, and mawveylong village which is awarded as Asia cleanest village. must visit, u can cover shillong in two days easily.