My understanding is that 2 males won't fight when there are no females present to fight over. I house my spare roosters together and they all survived the experience, even though I had a male and a female guinea in with them.
I don't know about the place you are going to, but at the fur and feather swap where I bought my guineas, the guineas were very vocal--- and the female has a 2 syllable cry that the males don't make. You can also ask the sellers--- they will probably be friendly and helpful if you are friendly to them.
As for their care, pretty much like chickens except that guineas are better flyers, and a bit on the wild side. It's best to keep them confined for at least six weeks after you get them so they know where their home is. Mine are confined right now because I want their eggs for hatching, plus I have predators around here and don't want to lose any. My pen is made of six-foot high chain link, it's about 10x12, and has a 'roof' of plastic chicken netting over the top to prevent them flying out. I put the netting up this spring because my guineas were flying out during the day and only going back at night, and I lost 2 in a snowstorm and was left with only 2--- luckily a male and a female.
And--- by the way--- don't worry too much about the guineas at the market being 'sick looking'. They may be just unhappy because of being caged and will perk up when they get to their new home.
