At the risk of this thread going far afield, IMO the 'series rating' clatrap has been blown way out of proportion, and is little more than an aggressively pushed sales tool. (It lets them claim 'exclusivre' rights to an entire job.)
I sometimes wonder about these things and recall that Cutler hammer (wetinghouse) was the big lead on series ratings. Although I suppose the up over and down method was working most of the time there were rare problems where two breakers racing to trip and the lower rated breaker failed. We have a secondary network in part of our downtown and the fault currents are astronomical. I had seen more than a few upstream breakers win the race to trip with the local breaker welding closed or open. Never had a breaker blow up Yet.
What I thought was a great benefit was that it brought attention to the hazards associated with high fault currents.
As for maker A breakers in maker b panels I know some breakers fit and some are approved and I know the difference and hope my staff do too.
