This is a little later (ca 111-112 AD, Bithynia), but it might still interest you:
X. 19 To the emperor Trajan
I beg your determination, Sir, on a point I am exceedingly doubtful about: it is whether I should place the public slaves as sentries round the prisons of the several cities in this province (as has been hitherto the practice) or employ a party of soldiers for that purpose? On the one hand, I am afraid the public slaves will not attend this duty with the fidelity they ought; and on the other, that it will engage too large a body of the soldiery. In the meanwhile I have joined a few of the latter with the former. I am apprehensive, however, there may be some danger that this method will occasion a general neglect of duty, as it will afford them a mutual opportunity of throwing the blame upon each other.
X.20 Trajan to Pliny
There is no occasion, my dearest Secundus, to draw off any soldiers in order to guard the prisons. Let us rather persevere in the ancient customs observed in this province, of employing the public slaves for that purpose; and the fidelity with which they shall execute their duty will depend much upon your care and strict discipline. It is greatly to be feared, as you observe, if the soldiers should be mixed with the public slaves, they will mutually trust to each other, and by that means grow so much the more negligent. But my principal objection is that as few soldiers as possible should be withdrawn from their standard.
Translated by W. Melmoth