My rule of thumb is to intervene when they turn into rolling, biting balls of fur.
Nipping is about the only way, other than shooting pee, to tell another pig to back off and get away.
Guinea Pig Introductions
The bath should calm them down a bit. Also, if you have a pen or something, put them all in seperate pens and let them be close, but not together. And maybe distract them with some veg.
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