Hi
You probably have done this already, but if you haven't, go to the Harmony Central User Reviews and look at what people have to say about the Barden's. It's been a while since i read them, but i seem to remember they are known for having big output (the pickups, not the reviewers!). You might encounter someone there who was combining them with other brands, and get some insight that way. Obviously it's a big investment to make and then find out afterwards that they are going to overpower any non-Barden pickup you have in there with them.
Is there any kind of Barden user group where you could make enquiries from that end of the equation I wonder?
There would be a way you could counter this issue if indeed it is an issue, though it's a bit weird....
go ahead and have the Fender pup in the middle and feed it, and only it, through a small booster, just a simple 1 transistor job....it would be between the middle pickup and the selector switch or controls, whatever the pickup would normally go to first. A small pre-set pot in place of the booster's regular control pot would be set to match the levels. Obviously it would need to be powered, most likely by a 9 volt battery. It would work fine but may not be an area you want to go into, I'd be the same, but it is a thought to throw into the pot here....
(BTW....the booster could change the phase of the pickup so that would need to be kept in mind when wiring it all up....a 1 transistor booster would turn the phase around 180 degrees...)