This of course poses a problem if you're modifying a tank that's already set-up and populated with fish. If this is the case you might want to introduce the Mollies but restrict feeding to every 3rd or 4th day. This should still encourage them to eat some of the algae while also providing enough food to keep the tank's other occupants alive. One solution is diligent pruning. Pull the plant out of the substrate, lop off the bottom portion, and then replant.
Roots seem to be easier to grow than new stems and leaves.
Other plants, such as Amazon Swords and Aponogetons, cannot be trimmed in this manner, but you can usually just remove those leaves that have gotten too large. Vallisneria can be snipped off at the top without damaging the plant. Simply pull up the plants you want to move and replant them in the preferred place. Most will recover quickly in a CO2 injected tank. After the break-in and when you're finally happy with the plant assortment and aquascaping, and your plants are growing like crazy, discontinue the CO2 injection.