Hi Soda,
Your route Tana - Fort Dauphin - Nosy Be - Tana can't be completed exactly that simply because there are no flights from Fort Dauphin to Nosy Be. So your route will be Tana to FD, FD to Tana, Tana to Nosy Be, and Nosy Be to Tana.
Since your trip is next year, I guess you haven't yet bought your international tickets? If you are flying into Madagascar with Air Madagascar then you will get 50% discount on all of your internal flights so keep that in mind when choosing your international carrier. If you are making a number of internal flights it can mean a saving of hundreds of dollars. Here's the link to the Air Madagascar page that explains it...
http://www.airmadagascar.com/offres/particuliers/nos-reductions-sur-linterieur
Good luck and have a great trip. You are visiting my two favourite places, Fort Dauphin and Nosy Be :)
The Air Madagascar website has a lot of information, albeit in French.
P.S. Edited to say, yes, from Fort Dauphin you can visit Berenty Reserve, Nahampoana Reserve, Sainte Luce Reserve and many other places. You don't say where else you want to go so I can't give you any further precision.