The Vertex diffusers are .5 to 1.5 CFM so you would need at least 3 of them. I'm pretty sure the last 2 digits on the model number of the pump is the CFM rating. In a 2660CE35-985 your CFM would theoretically be 3.5. I thought the lowest rating on the 2650/2660 was 3.7 CFM however. I don't see that particular model number on the Thomas website so I would double check the model # as well to be sure. I show the CE37, CE39, and CE44. I have the CE37.
I'd agree that for a "normal shaped" pond that size and a pump that size, the DE diffusers are probably more than sufficient, you shouldn't have any issues with getting all your water turned over.
CORRECTION: Vertex diffusers are .5 to 1.25 CFM.
