Thanks for your responses everyone!
First, I would consider getting another border collie because I do love, love, love them. But we have decided to be very firm with size (25 lbs or under). We have small cars and want the two to fit, and who knows what size house we will end up in (just moved out of 600 sf and I can't imagine having two big dogs there)! Max is a very large border collie-62 pounds-he's mostly legs, but I think he'll still fill out more. So a mix would be a good bet. We also want a dog that has those big melty puppy eyes that look up at you---max's eyes are, well they're intense- ha ha.
Second, thanks for the advice on mini aussies and their personalities, that is really important. I loved shiba inu's for awhile after seeing one hiking 14'ers, but after every single owner telling me what a pain they are I won't go near them. Plus, I'm finding mini aussies to be very expensive.
Third, I want to rescue a dog, I do. And if we can find a puppy then that is a definite option. Any advice on finding rescue puppies (I rescued an adult golden-cocker mix who was sweet but had aggression issues stemming from years of abuse)??
And finally, yes, Max is a total snob about other breeds. He does like herding types. He has liked a sheltie, and a blue heeler. He spent his first 6 months with our old roommate's 5 pound toy poodle and LOVES the tiny dogs. So maybe a miniature poodle. We want it to be large enough to keep up hiking.
And thanks for the kind words about making room for a second dog, there's enough love in this house for two.
-Hill