Bonjour isabelle25!
If you're flying from Paris, I don't think there are direct flights to Orlando (MCO) from there. So that means you have to change planes in USA, where you clear immigration and customs and catch your connecting flight, or change planes somewhere like Frankfurt (FRA) and fly from there to MCO (where you would clear immigration and pick up your bags).
Re layover in ATL or somewhere - I would suggest 2 hours as a minimum. I have changed in places like IAD, ORD, DTW, MSP, JFK, and ATL, and have found ATL and MIA more laid back and easier to get around, so my advice is to try for connections at ATL or MIA (Miami).
Someone on the UK planning thread had a query, and I posted this response, so have a look at this for info :
http://www.disboards.com/showthread.php?t=3158554
Don't worry about long delays/lines for immigration at MCO - you would have arrived at your final destination, and don't need to catch a connecting flight so who cares!
Although your query is transportation-related, strangely enough, I believe this question is better answered by us on the UK forum; there are even some French people on there, too!