I would like to say a couple of things....
Boys, if bonded young, usually will get on for life, girls are easier to bond, but once bonded boys usually don't fall out. I am curious as to whether you tried to quell the fighting, before labelling it as an issue and returning them?
I had 3 boys (now 4) who did fall out after almost a year, but i've kept guineas for years, and this is the first time we've had a permanent fall out, so it really isn't common. I now have another rescue, who has in the last week cost me over £100 in meds, as he came to me ill, but i didn't 'return' him. Guinea pigs are amazing creatures and i do wish you would have found them good homes, rather than just returning them should you find that they weren't for you as it is unlikely a guinea pig with severe diarrhoea as you described would survive without 2 - 3 hourly syringe hydrations, and possible syringe feeding, which i doubt the store would have done.
I'm sorry to be harsh, and i understand piggies are expensive, time consuming and, they do poop constantly too! Their vet bills can rack up quickly etc, but these are all things that should you have researched thoroughly you would have known, and i hate to see these amazing little animals written off so easily, when they have sooooo much love to give.
I'm not getting on at you - i'm just speaking up on behalf of all the gorgeous piggies out there