We land at CDG airport in Paris at 8:35 a.m. How long do we need between flights to go thru customs, get luggage, and catch our next flight to Rome?

Airlines? Separate tickets or all on one PNR? Which airline for arrival and continuation? Day/time of arrival? First class or coach?

Different airlines and tickets
Arrival-Air France July 3, 2010 morning
Continuation-Alitalia July 3, 2010

You will be much better off if both flights are on the same PNR. AF and Alitalia are both SkyTeam airlines so that should not be a big deal. Contact AF and ask that the PNR's be merged.
As for transit time, no less than an hour, preferably more.

Sounds like it is too late and the two tickets are already issued (and there is no way to merge two PNRs) so the OP has to treat the situation as two separate trips.

An hour for that change is stretching it. Alitalia has flights at 09.45, 10.10, and 12.30. Forget the 09.45. The 10.10 is really, really tight so I hope the OP is on the 12.30.

Even with separate tickets you can check your bags through all the way to Rome. Just be sure to give the AF agent at checkin the connecting flight info.

You won't clear customs until arrival in Rome, just immigration in CDG.

Right - merge is not the correct term. Ask that they be linked, which is possible.