There are plenty of people who only speak their mother tongue and Urdu in Quetta. The biggest threat for the Baloch community is their people learning Pashtu and it becoming more widely used. Im pretty much sure a number of pashtuns, Brahuis, Baloch understand one another but they always prefer to use Urdu with one another.
Quetta is mostly pashtun and then theres large numbers of baloch, Brahui, Persian speakers and 100,00 Punjabis.
By Spocks definition Balochi should be imposed.
I would say that Quetta is different..It's as if every one understands each other. You can talk brahui/balochi to a pashtun and he will understand it. He will perhaps reply in Pashto and that's also understood vice versa. There are many languages and dialects in Quetta and people have neighbours who are of different language...as times goes by the languages are picked up. I've also seen it from my Punjabi friends in Quetta who are fluent in Brahui and Pashto as I understand them when speaking in Punjabi