Yes, Sir Douglas Quintet and other Texans worked in California in the late 60s, but that does not make them Country Rock.

Texas Rock like the Flatlanders equals anything that came out of any part of the US. Often called Alternative Country, hard to put boundries around that also.

The Nitty Gritty Dirt Band is now listed as begining in 1966. Used to be listed as 1965 but the name and musical style were different.

The nomination of Buffalo Springfield is perfectly on the mark. Many Country Rock pioneers came from there when the group broke up.