Do you think that speciation is occuring within domesticated dogs?

Dog breeds all readily reproduce. (Except perhaps bulldogs, who can’t even reproduce within the breed without human intervention.)

No, I don’t. When dog breeds are allowed to mingle indiscriminately, they tend toward one prototype dog.

How many Chihuahuas have offspring with Great Danes?

Nevertheless, are you talking about morphological or genetic speciation?

It can reduce the genetic variety that’s important but I don’t think it will make them incompatible.
As a side note, someone in NY shot a huge coyote, they thought. Turns out with genetic testing, the identified it as a wild wolf.

It’s an interesting premise but if there were any truth to it, I’d expect the shelter population of mixed breeds to be decreasing. It isn’t.