Hi Chethu..
Looks like a brilliant collaborative effort.
Few things I can think of :
How many days you have?
How adventurous are you planning to be ?
650 Km by straight route
We have riders like Vikram -Rocky- who would dust it in 48 hours !
So the daily distance covered will depend on your objectives.
December, especially Christmas time, will be busy in Kanyakumari. Hotel stay should be better booked ahead.
Hybrids are easier for long trips but all combinations have been used ..so no advice on that !
Not sure about safety plan..riding in dark early mornings /late evening would not be considered safe ! Daytime December is tolerable ride down south from heat point of view..January may be even better.
But again...each rider has his or her comfort zone
Fluorescent / reflectives strips on yourself and bikes a must.. lights and spares too
Warm regards and all the best
Ali Poonawala