You can.... The better question is why would you want to. I guess if you were adding to a system that already had 206 cards installed, adding a 308 makes some sense. There's 206 cards out there that still work right?
Good 308EC cards are dirt cheap on flea-bay, and a local used telecom parts dealer in my area sells them for around 25 bucks. They only resell the 308EC R3.0 cards because they have improved fax CNG detection and caller ID characteristics. Just my 2¢.
But, yeah. Like Dex said, you can mix them without any problems.