I would not trim cardigans fur short during the summer months. Corgis are not meant to be trimmed that is why they shed. The hairs will not grow back the same! There should be another discussion here about shaving corgis, not reccommended. You should be able to find it if you look under forms I believe. Their fur protects them from the heat and the cold!
*nods* The best kind of grooming you can do on a corgi is de-shedding. The FURminator is AWESOME. I used it on all three of my cats to test it out. You would not believe how much undercoat came out, and how nice, shiny, and defined their coats looked afterward. I can't wait to try it out on Banzai.
Sadly we put it in a box, when we moved, and don't know where it went. T_T