If we "need" to know available fault current .... have you tried doing any 'arc-fault' calculations?
The question matters, because you ultimately arrive at PoCo equipment,and you wind up assuming infinite current available.
So, the switchgear has the first role in defining your available short circuit current. Add some wire length, and the next breaker in the system now has a defined short circuit current - regardless of the PoCo supply.
Sure sounds to me like something you have to do as the first step in a selective coordination study. There you go, off to look at the same response curves.
In truth, what I think the manufacture is telling the OP is that his gear has bussbars that are rated only to 10KA. If so, these ratings may be misleading. I seem to recall (from testing lab experience) that the industry practice is to mount everything to 25KA standards, for their own manufacturing convenience; the 10KA marking is often pure fiction. A chat with the gear maker might clear up this point.
Don't overlook the role of the feeder wires in limiting SCCA. While you're talking to Bussmann, you might want to ask them why they do their testing with 12" leads
