I think the agreement not to hire each others staff is an urban myth.
I used to think the same thing until a chief pilot of a particular airline told me personally that a chief pilot of a regional airline had rung him in regard to the high number of his pilots being hired from them and could they come to some sort of agreement.
On saying that:
1. Owners of GA/Regional airlines tend to over estimate their pulling power in the major airlines in Australia. I remember one GA owner in QLD who threaten people with their careers at a particular regional airline if they dare left before his nominal time frame.
2. I think that even if there was a gentlemen's agreement in place, if there is large expansion, all bets are off.