MawBroon
Personally, I don't think a failure to get on with someone would be toxic. Then again, due to my personality, I tend to get on okay with most people. But I could see how, for example, if you had two people with very dominant personalities it could be difficult.
I would define "toxic" more as things such as passive-aggressive behaviour, outright aggressive behaviour, attempting to "parent" an adult child/CIL, that type of thing.
Those kinds of behaviours are not acceptable. I think in this case it's reasonable to say in the moment, "I am bothered by your behaviour and would like it to stop." If the person refuses, then you change the subject or, if the behaviour continues or is more serious, you leave.