I don't think the Army would allow another major army to form out of the Ministry of Interior.
The correct approach would be to re-organize FC, Rangers, etc, into a 'National Guard' and then place said force under the control of Army GHQ. In turn, Army GHQ gives the National Guard one mandate: manage the internal security affairs of Pakistan. The advantage of this approach is that the National Guard won't be seen as a threat to the Army, but at the same time, could get its own attack helicopters, transport helicopters, etc.
BTW, this wouldn't be unprecedented in Pakistan. The PN already runs the Maritime Security Agency.