I'm setting up an aquarium as we speak, and as you see in my signature, I have a 2213 and pressurized CO2.
I've done tons of research on this topic, and found quite a few people who are running CO2 directly into the inlet tube of the 2213 (http://www.google.com/search?sourcei...im+classic+co2
A couple of things to keep in mind:
-some people drill a hole in the intake siphon (hard plastic j-tube) and insert their CO2 tubing there. Suppose for a second that your power in the house shuts off, and your CO2 keeps bubbling into the tube. Soon your siphon suction will be broken, and when the power turns on, you will run the cannister dry (big problem). I've determined that I will run CO2 tubing down to the strainer basket so that the suction has to pull the CO2 bubbles in the strainer--this way, if the power goes out, the CO2 will bubble harmlessly to the surface of the water. (Caveat--if you're rimmomg a solenoid on the CO2 regulator, if the powergoes off, the solenoid will shut off the flow of co2.)
-Re: accumulated air in the cannister--sounds like you've read the same things I have about this, namely that the Eheim Classics don't tend to have a problem with CO2/air amassing at the top of the filter.
I'll let you know how it goes--I should be done painting my DIY stand & canopy this weekend, and will set the system up after that.