If we arrive at CDG from on British Air (already booked) and fly on to Geneva (probably on Air France), will we go through Customs in Paris or Geneva? How long should we allow between flights?
Customs in Paris if you have nothing to declare is nothing. You just walk out the door. I am assuming that the flights are not protected -- meaning bought on one ticket or itinerary. In that case, you will have to go through passport control, collect any checked luggage, exit the secured area and then check in (unless you are able to do that inflight from the US or before you leave), or check your baggage for your ongoing flight. I don't like unprotected connections but at least in this case, the cheaper trip is the one at risk if anything happens to delay your initial flight. Passport control can take a long time at CDG unless you have access to the priority lines. Although, you could be done in less time, I would allow 4 hours between estimated time of arrival of the BA flight and check in time for the Air France flight. In another thread, I suggested 6 hours for an unprotected connection between flights but that was for Newark or JFK, which, in my experience, do not have good records for on-time arrivals.
It depends on how much of a gambler you are. There are a lot of reasons why you could be delayed before you check in for your Geneva flight. To name a few, mechanical issues, bad weather, holding pattern for landing, long lines in the airport. If you only have carry-on luggage, at least you don't have to wait to collect bags and they won't go astray - just filling out a lost luggage form can take ages. If you miss your flight, you're more than likely out of pocket and have to buy full fare tickets on the next available . . . Of course, if everything goes smoothly and you've allowed plenty of time at CDG, it'll be a long wait for your next flight.