Hi ! thanks for all your comments
So our trip last two weeks, I went there with four of my friends that are not as crazy as me, so it wasn't a herping-only trip, we also went sightseeing, snorkling in Nosy Be... so I would say that we did only 7 days of full herping.
I can only confirm what Bastien said, this island is crowded with reptiles, it's ridiculously easy to see that many species. It's very easy to herp, the best thing is to go in parcs and reserve, and there you can freely go around, even leave the paths if you want.
I organised this trip with a driver-guide, it's the best thing to do I think, as for safety issue, going with a good local guide who knows where are the danger is the best (even if Madagascar is pretty safe once out of the big cities). He also help to communicate with the locals in the villages, with the parcs guide etc... He rented a big 4x4 and took us anywhere we wanted.
The struggle with epidemics is located in the big cities, once you're out in the parcs there's no problem. Crime is present, but with a good guide who knows where not to go there's absolutely no risk.
So I highly recommend you to go there ! if you want more advice or the contact for my guide (He mainly speaks french, also english but not very well) I'll happily help you