We have a house between Montpellier and Capestang, so I do that exact trip several times a year.
For several reasons, I usually take the TGV directly from CDG to Montpellier and then just rent a car at the Montpellier train station. I usually fly United from Dulles to CDG, and therefore arrive in Terminal 1 at CDG. Because I am usually carrying a bunch of extra stuff to take to our house, I don't want to deal with the hassle of changing airlines and rechecking my extra baggage or getting into Paris and taking a TGV from Gare de Lyon.
If I were traveling lighter, Air France has several flights a day from CDG to MPL. If I wanted to stay for a couple of days in Paris at the front end, I would probably just take a train from Gare de Lyon to Montpellier.
In either case though, I definitely want a car while there. Rentals are not hard and not expensive. I have never tried a taxi, but they are available.
PM me and I would be glad to discuss all of this with you.