There are some pigs that will just always clash or will never get along--just like people. But, I am a little confused and have a few questions about your introductions.
How old are the 2 pigs that seem to have the trouble with each other?
Older pigs tend to be dominant, and "teenage" pigs going through puberty like to push their luck --they think they are all growed up now and want to try to be boss.
What do you mean by 'hurt'?
It is normal for gp's to chatter, mount, argue, and even for some fur to fly. As long as no blood is being shed, you shouldn't really intervene (introducing and reintroducing over and over only stresses them out and makes the process start from scratch--the pigs don't remember 'oh yeah that is that guy that was here yesterday')
Where are you introducing them?
Introductions should never ever be done in the cage where there are already established scents. You need to place the pigs in as large of a neutral area as you can provide, away from the cage. Refrain from adding any hiding places in the introduction area (so no pigs get hemmed up with a ticked off pig) until things have settled down. It is a good idea to add piles of hay, and veggies in the introduction area so that they can "discuss things over dinner"..it provides a bit of a distraction. Monitor the pigs, and keep a dustpan or towel nearby to break up any true fights, but as hard as it is, try to refrain from stopping introductions and pulling pigs unless there actually is a rolling drawing blood fight at hand. It is hard to watch sometimes and is nerve racking, but "argueing", bullying, and even nipping is a normal part of the process.
How big is the cage you are adding the 7 pigs to?
Boars are territorial and dominant. If you don't have adequate space for them, they are going to hate each other.
You should add 2-4 sq ft for each additional boar, so a minimum size cage for 7 boars would be 19 sq ft--2X12. More would be better, but anything less is probably asking for trouble.
While you are intorducing in neutral territory, do a thourough cage clean. This means clean out bedding, wash the insides, as much of the grids, and all the toys, dishes, and hidey houses with vinegar/water to eliminate any "boar scent". If the two pigs still just do not get along, it might help to put a divider in the cage--place maybe 2-3 pigs in one side, and 4-5 in the other side--make sure the two that do not get along are on opposite sides. Leave the divider in for a week or so, so that the pigs get aquainted, but don't have access to each other. And then retry introductions in a neutral area again in a week or so. (another cage cleaning will be in order before you introduce them back in with no divider).
Here is some more info and links from Guinea Lynx
Be sure to read over Teresa Murphy's page on Social Lifefor her advice on how to improve the odds of a successful match, introductions, and much more. It is a must for anyone planning to add a new pig to the mix. And for proof that boars really can get along with each other, check out all the success stories and tips that Charybdis has collected.