Gen Mitha was selected because he was para qualified and Gen Ayub Khan knew him to be a competent officer by way of Yahya Khan. After he was asked to take up the job, Mitha too went to the UK and attended training with the British SF including SBS and then later in the US with some unspecified special forces.
I think to raise a unit from scratch, you need to have officers with not just the practical experience of the operator, but also administrative and command abilities. Perhaps some favoritism had a part to play, but Mitha was popular with not only the officers but also the men who served in 19 Baloch (which later became the SSG).
I know of officers who served with Gen Mitha or joined the unit right after he had departed SSG and they regarded him highly as a good, competent commanding officer.