Hello and welcome to BYC!

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Yes, letting them "duke it out" has a high probability of this resulting in the death of the bantam.
It's now too late to quarantine her so that is out.
Can you get a second bantam from the same place you got the first? Having 1 bantam trying to integrate into a flock of PBRs is asking an awful lot. PBRs are pretty dominant birds.
You will have to leave the bantam(s) in the "look don't touch" arrangement for several weeks until things have calmed down a bit.
You also need A LOT of space for a successful integration. How big is your coop? How much roosting space do you have in inches? How big is your run? Do you have multiple feed and water stations? Are their various height perches in the run? Are there places to get out of sight of the PBRs in the run?