I'd schedule a taxi for 6. I know taxi G7 has Maxicabs (vans) for 6. Much easier than trying to research stairs, times and connections, especially if the weather isn't great that day. G7 has an iPhone app.

you'd better try this link known as Paris metro route planner
http://www.transilien.com/?siteLanguage=en
have a nice time in Paris

Everything related to transportation in Paris is here:

www.ratp.fr

You can schedule a taxi pickup online or by phone: http://www.taxis-bleus.com/

I would also check out the taxi van. It would make for a much easier transit.

I am a budget traveler but on our last trip to Europe my back bothered me a bit for a couple weeks and I tell you, I was sold on the whole taxi thing...loved it. Easy breezy!

Taxi for sure. I'm usually a budget traveler too, but IMO some economies are false economies . . .

I agree with everyone who said take a Taxi and especially janisj.

Thanks for the feedback! St Cirq, I'm bookmarking the Ratp.fr site... somehow I forgot all about that planning function.

I'm starting to lean towards a taxi, just to make it easier on ourselves.

Does anyone have an idea what the taxi would be from the Invalides to Gare Montparnasse? Do the 6-person vans charge at the same rate as a small 2-3 person taxi?

Jeez, you don't need to 'plan' metro connections. You just look at the map and follow the signs.

The G7 Taxi website has an estimated fare page:

http://www.taxisg7.com/estimation-tarif-duree-course