The people I got ours had 2 in a 10 gallon one. She did not have them long...She had got 2 , and the female was pregnant. She had 3 babies. She put the male in another 10 gallon aquarium. We got one of the female babies, and someone else before us got another. About 2 weeks later, all of hers died. I think it was because they were in an aquarium. It is hard for an aquarium to air out. Especially for multiple ones in one cage. I don't think she changed the bedding very much either. Anyway, all of hers got colds and died. Ours was ok when we got her . Ours never had a cold. She is a also a very spoiled fuzz ball.
Anyway, sounds like if you do get the Guinea's, it will be like a rescue. It does not sound like the present owners are taking good care of them.