Your fastest way from CDG to Gare St-Lazare would be by RER, not taxi (no morning rush hour traffic to contend with). RER B to Gare du Nord, a walk through the connecting tunnel to Magenta, and the RER E to Haussmann St-Lazare. Fare: 8.20€.
If your plane is on time, you get through immigration/passport control fairly quickly, and there isn't a long line at the ticket window at Gare St-Lazare, you have a good chance. You'll have a better chance if you are able to self-print a Prem's ticket by booking in advance at www.voyages-sncf.com. However, that ticket is non-refundable and non-exchangeable. If you experience any delays, you'll be out the cost of the ticket and you'll have to buy a new standard fare ticket for the 12:25 train.
The safest thing to do would be to plan on taking the 12:25 train, using either a Prem's or a standard ticket. Use your free time when you get to Gare St-Larare to buy the fixings for a picnic lunch to eat on the train.