You are right in not wanting to add a third boar into a pair, although some people can have success with it (I have a number of times, but also had some colossal boar trio and pair failures!) you need to know the characters of the guinea pigs exceptionally well, nor would I try it with anything younger than about 2. There's also the added that you will need to increase the cage size and in my opinion a trio of boars should be in nothing smaller than 6x2 ft. And thre benefit of having done lots of guinea pig introductions helps. It's far better to be safe than sorry.
Personally if I were you I'd get another couple of boys, because I love boys. However pairs of neutered males & females are also lovely and tend to have a very close bond. You can keep pairs of females close to pairs of boys and not have issues. I have males and females in the same shed (separate cages though) and I've never found it to cause much aggro amongst my male pairs or trio's, but it does seem to frustrate sows who have particularly strong seasons!
Cage stacking should be ok as long as they are very secure, what kind of cage do you have, intend to use?
