8 days France-Switzerland Tour

I'm a bloke, for the record :)

Should I take Swiss Pass or any other pass?

Should I book hotels in Montreux and Interlaken (or nearby areas)? As there are very few hotels and very expensive too.

Apart from Paris-Geneva & Bern-Paris trains and Choclate train, do I need to make any other bookings in advance?

Yes, you should book hotels. Have a look at booking.com. Accommodation in Switzerland is not known for being cheap, especially in cities or popular tourist areas, and the currency exchange can make it feel worse.

Definitely book your tickets from/to Paris in advance. Swiss rail tickets are not cheaper in advance. You can look into passes on Swiss Rail: http://www.sbb.ch/en/travelcards-and-tickets/railpasses.html

For accomodation in Interlaken, check out www.swissbackpackers.ch - don't let the name fool you, these hostels also have double and single rooms, and not just dorms, so it's worth looking into if you are hunting for budget accomodation. and try to book as soon as possible. Your intinerary is a little rushed but you are using your time well. Distances in Switzerland are not massive so you should be able to manage all you want to do. I would suggest if you want to relax, the boat trip on the Lake of Brienz is shorter than Thun so it will give you more leisure time afterwards before your train to Paris.