Boars are full of character we have pairs and trios of boars that all get on well. Boars can fall out over dominance (especially when they go thorugh their teenage stages and if they can smell ladies) but if boars are provided with plenty of room a food bowl/water bottle/house each we find generally they get on really well. they have the odd rumble to re-enforce the pecking order but they generally co-exist peacefully. Our trios are in 6ft by 2ft housing and get on great!
Always have a spare cage handy though just in case if you are keeping boars once a pair is seperated it is hit and miss whether they will be re-introduced successfully.
Adults introduced to young boars has works well as long as you keep an eye when the younger boy gets teenage, but mixing adult boars is a very difficult process, neutering and pairing with a lady or pairing with a young boar are our first choices.
Sows can be great in groups, we have had a few stroppy ones who are very dominant but generally they accept each other well. Some sows mix effortlessly but we have had a couple who definately did not like new arrivals, my one and only guinea bite was from splitting two girls having a fight the minute they met!
It all really comes down to the personality of the pig not what gender they are, if areas of competition are removed with ample food water, hiding spaces and room most guineas will get on, some tolerate each other others fall in love and are inseperable.