It might be worth a try introducing them in a neutral area so none of them feel they have to protect territory IYSWIM.
Give them lots of distractions in the form of food - dandelions, kale, spinach...anything to take their minds off the fact that they might fight.
Watch them closely for signs of dominance/unease that could lead to a fight, such as teeth chattering and holding their heads up (I'm sure you'd have seen this behaviour before!) and as long as it doesn't escalate into a ball-of-flying-fur, then you may be alright.
Expect some little spats as they sort out dominance and hierarchy, but as a rule of thumb, I say if they're not drawing blood then let them get on with it, however, if they are obviously stressed out, separate them and try again in a week.
If this doesn't work, then try putting them all in the bath together and bathe them - sometimes the terror of the bath will make them forget their differences, plus it'll mean they all smell the same.
Also expect a bit of urine spraying to happen [yuck emoticon] as this is all tied in with displays of dominance.
Another thing you could try is to put them in a cage separated with a wire mesh, so they can smell, see and interact with each other through the mesh but can't get close enough to cause damage to each other.
I am assuming you have boars though, so I apologise if you have sows as I'm not sure the teeth chattering and head posturing is applicable (and they don't try to urine spray either).
How old were your piggies when they started fighting?
HTH and good luck!