It's difficult at times because as the DC get older there are times they're reined in by different people; DS13 is very sporty so on the pitch (rugby, cricket and football) coaches can raise voices and things can get heated between players. He also does a martial art and again there are moments where he's had to be told off; it's rare, but I'd never dream of stepping in and telling coaches that they can't raise voices or shout at my precious firstborn (unless it was totally unreasonably handled and it never has been so far).
At home, Ex and I parent DS13 similarly, as do Ex's DW and my DH; none of us is particularly shouty or aggressive, so if he's rude or misbehaves occasionally, all it takes is a word of warning and he'll bring himself back into line. DS8 is much, much more feisty and has a fire in his belly that DS13 doesn't, so there are times both DH and I have raised our voices at him in ways we'd never have needed to with DS13.
Ex has told off DS8 on occasion and whilst he's never realised his voice, I'm ok with it purely because I trust Ex implicitly to help 'parent' DS8 positively.
Physical violence or aggression is never acceptable in any form. Grabbing, pushing, shoving, rough-handling, any of it and I'd have no hesitation in wiping the floor with whoever it was. It's not how I've raised the DC, it's not how we parent and it's not something I tolerate. I don't want them to be afraid of me. I don't want to rule through fear - I want them to do the right thing because it feels good to make good choices.