The problem with this question is that what monks did and what they wrote that they did are not exactly similar.

We know that monasteries usually relied on the protection of a local lord as well as the inviolate nature of the church. My field is European monastic history and I've never seen any textual mentions of hiring mercenaries, but there is significant disincentive to include that information. Putting your trust in a paid cutthroat is usually frowned upon when you're supposed to be putting your trust in God.

With this in mind I would be very surprised if they did not hire mercenaries or at the very least that they did not gain the local lord's protection from time to time with large "donations". As far as I know, however, there is no proof.