Whoa!
I have had negative Birth weight EPD bulls used on well grown out high performing Hereford and Hereford red/black baldy heifers that were well manged by people that calved out a few hundred head yearly with no problems. We did not recommend they do this but they used the bull the previous year and felt we was easier calving than the Red Angus bull they bought for the purpose. This bull had a Birth weight EPD of -2.9.
Before we like to see a bull we sell used on non Charolais cross heifers we want to know more about the operations lay out, their management, how the cattle will be watched at calving time, the size the females will be at calving and history of any calving problems in the herd. Only if we get positive vibes on all these point will we tell someone it is OK.
Generally when you go to a bull sale the bulls by definition are unproven. A bull with a -4.0 Birth Weight EPD may turn out to be much less easy calving than he seemed. Conversely a yearling bull with a positive +1.0 EPD may prove to be a sleep easy bull in the end. This is why before I do an experiment I would want to see at least one calf crop off your COWS to see just how easy calving he really is. It is this reason why well proven AI bulls like Oakdale Duke (-3.7 EPD) or Big Ben (-5.2 EPD) are preferred for doing what you are proposing. Remember you have to know the calving history of the herd. A negative birth weight EPD isn't everything here.
If you have to buy a bull go to your local Charolais breeders and tell him what you want to do and gauge their reaction. You will learn plenty from them on what works in your area and management.
I would recommend considering a calving simangus or balancer here. Why? You have no Angus in these cattle meaning you will get a dose of hybrid vigor from the Angus and continental influence both. Plus I think calving ease bulls should be easier to come by in these breeds, as Charolais bulls are becoming expen$ive.