The Pak Army regiments are not based upon ethnicity. Baloch Regiment is the most decorated regiment, and most soldiers that graduate prefer to join the Baloch regiment. I am Pukhtoon and family is in Sindh and my uncle was in the Baloch regiment. There are also alot of Baloch in the Punjab Regiment.
So basically your origin/ethnicity has nothing to do in which regiment you end up with. On graduation they ask for your preferred regiment and then depending on availability you are assigned to those regiments.
The percentage of the different ethnic groups in Pak Army as of 2007:
Punjabis: 53.19% ( this includes Saraikis )
Pashtuns: 21%
Sindhis: 13.5% (includes Mohajirs, who predominately prefer to join the Navy and Airforce)
Kashmiris: 9.11%
Balochis: 3.2% (Balochis predominately prefer to join the Navy)
http://404ihpakistan.weebly.com/pakistans-military-ethnic-composition.html