Zim is landlocked and surrounded by states that would resent mercs or their logistics crossing their territory.
Logistics is impossible, the nearset port is 25km from the eastern border of Zim (Biera in Mozambique)
Mercs were used in Sierra Leone but they were hired by the government and Sierra Leone does not have the logistics problems that Zim would present.
The mercs were cutting a swath through the RUF insurgents until the UN intervened and forced an agreement brokered by Charles Taylor of Liberia and Jesse Jackson of the US that set up a "government of national reconciliation".
Mercs could deal quite handily with the Zanu PF cadre. They are thugs, trained ionly in thuggery, rape and murder. But Zim has just brought back 9,000 regular army from the DRC where they have had the benefit of a prolonged live fire exercise and 3,000 more are soon to join them.
As soon as mercs are detected moving in, especially mercs hired by whites, the whole of Africa would converge on them like white cells to an infection.
If there is to be an insurrection, it has to be one that is mounted by indigenous peoples, having an indigenous cadre and supported by African governments.
In fact, without the acquiescence of Mozambique and South Africa, it is simply not going to happen.