Breeds that can hold their own against a heeler?

Before anyone asks, no, getting another heeler isn't an option. I love my boy and my mom loves him (I swear, he listens to her more than he does with me, like she's "grandma" and thus more worthy of respect :|), but the next dog we get will technically be hers and she absolutely does not want another heeler because of how rough raising this velociraptor has been so far.

Her ideal next dog would be a chihuahua or yorkie (both of our previous dogs) as she would be able to carry a dog that small. That isn't really an option she's willing to go with though because she doesn't like the odds of a 6 to 10 pound dog playing with a potentially 30 pound dog. Not that he would hurt them intentionally, but we all know accidents happen and with the size difference, they could be more likely to happen.

So to the question at hand. Does anyone have experience with (preferably) non-herding breeds that can hold their own against the play style of a heeler? On a scale of 1 to ACD, how difficult are/were those breeds to deal with as puppies? I personally don't have much of a problem with having another dog in the house who's on the higher end of that scale, but again, this is technically going to be my mom's dog. As long as we have a list of breeds to look into, we can go from there with research.