I use Jamie Hanchar at Doggy Zen Den (on Ridgeview near the Custer/121 intersection). They do require that puppies have all of their shots (including both Bordatella boosters). Blaze went at 6 months, but I wish I had started him sooner...around 4 months might have been better for us.
I noticed that you are looking into agility training. Have you settled on a trainer for that? I live in Plano, also. We plan to do herding training next month and then start with agility training in McKinney after that with Blaze. They recommend you wait until Corgis are 1-year old before starting high-impact agility. It gives time for all of their growth plates to set.
I have just started looking around. I had looked into herding as well but wasn't able to find anyone in this area. May not have looked good enough. :) I just want to get him into something that will help us bond and challenge him. Let me know how your herding traning goes. I am new to this all and am trying to take it all in. I also have German Shepherd that is a little older that I would like to try and do something with as well. Is the trainer you use for the herding breed specific?