Hi,
Rail passes are rarely a good deal in France. Have you looked at the cost of the journeys you plan to take, and compare with that of a pass plus 10 euro per reservation (mandatory on most routes)? Check oui.sncf/en, Trainline.com or loco2.com to get fares.
You left it too late for the very best fares (released three months in advance) , but I just had a look at Paris-Bordeaux and I'm still seeing good ones at 60€ for the regular trains and a 19€ "Ouigo" low-cost train (check luggage restrictions and check-in deadlines carefully) that looks like a great deal if your foreign card works for it. Likewise, Bordeaux to Marseille is just 32€ on several Sep 26 departures.
If you insist on getting a Eurail one-country pass FOR France, you must get it in advance, so order it soon to allow for delivery. You will then need to 'activate' the pass at a railway station office in France. And yes, flexible versions (X days within a month) do exist. You will then need to purchase reservations, either via Eurail, Rail Europe (an agency), or at a station in Paris but you will queue.
But frankly, I think that buying point-to-point tickets will come out both easier and cheaper; just book your tickets as soon as your route is fixed.
Unrelated: what are you planning to do for five days in Bordeaux? Unless you have friends or family there, it's a lot.
Edited: I remember your other thread, with your interest in wine you shouldn't stay in Bordeaux all the time, you should try to spend a night or two in St Emilion or similar, to get closer to the vineyards.
