Are blue heelers good with other dogs?
Of course the real answer is "It depends on the individual", but as a whole, Australian Cattle Dogs are good with other dogs.
Like every breed, Australian Cattle Dogs have behavioral tendencies. You could say that they are generally high-energy, loyal, intelligent, and protective where a retriever might be more generally described as friendly, playful, and affectionate. Due to the heeler's protective nature, they can be assumed to be aggressive or unaccepting of other dogs, but I have not observed this to be the case.
Dogs are individuals, and as dog owners, it is our responsibility to teach them what we expect. If you are considering getting a puppy of ANY breed, you have the opportunity to ensure that they are good with other dogs by exposing them at a young age. Take them to dog parks, and consider a "doggy day care" a few times a week where they are socially exposed to other dogs without the pressure of having to "protect" you.
If you are considering adopting an older heeler and you are unsure of their individual temperament, ensure that you are able to do a meet-and-greet with any current residents in their new household (this includes other dogs, and especially children.) If they are in a foster home, the host family will happily share information about their interactions with any other members of their household - human or otherwise. If you are considering adoption from a shelter, request to arrange a time and place to do introduce your current pets to the potential adoptee.
My personal experience: when he was a puppy, my heeler loved playing with other dogs. He was exposed from a young age to friends' dogs, the dog park, and doggy day care. Now that he is a little older, he is largely indifferent to other dogs. Even if I take him to the dog park on a busy day, he's more interested in chasing a ball than in playing with another dog, but he does have his BBFs that he still looks forward to a good wrassle with!