Flight from Paris to Venice

I always book through to my final destination on the same ticket that I'm using to get to Europe. That way, if the transatlantic flight is late, it's the airline's problem to get me to my final destination. If you can do that, it would be the safest approach, and it's probably worth the risk of the short transfer time.

The transfer time you're talking about at CDG is very tight if you are changing airlines, particularly if you end up being forced to check and retrieve your bags. In that case, or if your arriving flight is notably late, I don't see any way you'd make it.

We flew from San Francisco to Paris, Paris to Venice in 2010. We had a 4 1/2 hour layover in Paris. I remember it seemed very long. I can't imagine what a 7 hour layover would feel like. However if your first flight is delayed and you miss the second flight I don't know how difficult it would be to get another flight. I would be tempted to take the short layover. Maybe others will feel differently. Good luck.

Is there a reason you are not just flying straight to Venice? FF miles? Flying nonrev? Just a fantastic deal?
It makes a big difference what airlines you are coming in on and departing. If you had to make a transfer from terminal 1 to terminal 2, 1 hour is impossible. You just might find 6 hours is fine, a nice window if your incoming flight is delayed (it does happen), time for a leisurely early lunch, no hair raising time stresses. Anytime of the year its busy at CDG.

Agreed that it's best to book the whole journey on one ticket. If an agent helping you with a frequent flyer award can't find seats available now on a code-shared connection, then ask if they can add it later. Or they may be able to find connections that you did not see online.

Here's my take (many of these points have already been made in the posts above):

1. You should have a REALLY good reason for not booking this all on one ticket. (I just did something similar to Sicily through Rome, for frequent flyer mileage ticket issues; otherwise, I wouldn't dream of it. And I allowed 3 hours for the connection, and even then I bought a second back up ticket for 7 hours later, just in case).
2. If you are on separate tickets, you will have to go through immigration, then pick up all your luggage, then check in for your second flight, then go to the next gate and go through security again. There's NO WAY I would allow only 70 minutes for this; 70 minutes is very tight at CDG even if you are all on one ticket, but at least that way you're protected (the airline will put you on the next available flight to Venice at no extra charge).
3. CDG is always very busy (it's not just a tourist airport, so the fact that you're going in March doesn't help you), and in addition is huge. To give you some idea, Terminal 2 is divided into sections 2A through 2G. Getting from one part of the airport to the other can take a long time. And as said above, while staying in the same terminal is bad enough, if you have to change terminals, it's far worse.
4. If money is no object, you can always buy a last minute ticket from Paris to Venice. But look at the prices for this now, to make sure you can afford it, before you make it your back up plan.
5. If this is about frequent flyer availability, do call; Laura's right that sometimes they can give you tickets you can't get online. Even though there's a fee for booking these, it's far easier and cheaper than the alternatives.
6. If you are flying on mileage tickets and can't get ticketed to Venice, look into doing what I did for my Sicily trip. While the advance tickets are still affordable, buy two from Paris to Venice several hours apart (in your example, buy both the 720 and the 1300). Just consider it part of the total cost of your trip, as you'll use one and throw away the other. But at least this way, you're covered. If that's too expensive, then follow Sam's advice; buy the 1300 one, and plan on lunch at the airport.