I would stuff that corgi in the chicken coop, or in the kitchen with a chicken or two as soon as you get it. Sit there with a chicken and the dog and tell it to be easy with the chicken whenever you catch him being rough. I am a swatter, if I catch a pup doing something it ought not to I fly up out of no where and swat its butt and say NO! really loud. The swat doesnt hurt, its just shocking. Its the same effect as other pack members roughing them up when they get out of line. Except they actually bite them.
Make a huge-butt deal out of anything good it does. If he's gentle with the chickens tell him thats a good job he did. When he's not easy, tell him no and shame on him. They'll pick that sorta thing up wicked fast.
I had a 2 year old lab/husky rescue that I taught to catch my free range chickens for me. She'd just pin them down with her front feet and hold them there without using her mouth until I got there to pick it up. She did it on command and otherwise was never allowed to touch the chickens. You can train a smart dog to do anything. Corgi's are pretty intellegent, any herding dog is. Theyre problem solvers.