Others seem to cache with small dogs, but my concern would be that small dogs may get tired too easily or have difficulty in certain terrains. Others probably could comment better? My friend has a jack russell terrior and it can only go maybe 2 miles before he needs to be carried, kind of a pain I think, but he rides well in a back pack.
I have a black lab mix, he's 10 months old:
www.dogster.com/?225918.
He is a great caching partner:) Keeps all the bunnies out of my way! Of course, today we did a 3 terrain cache and it wore him out. While I was climbing over the side of the mountain he sat on the trail and watched me. About the first time since I've got him (at 5 months) that I've ever seen him tire out though.
Corgis are adorable though, hope you have fun with your new addition, I'd suggest rescue if you are open to that, if not please make sure you go to a reputable breeder, backyard breeders have a tendancy to produce dogs that have issues.