Guinea pigs are very sociable and should rather not be kept as singles. If there is no other way the human owner should try to substitute the missing mate as skillfully as possible. Female gpigs usually get along with each other well. For single males only female mates should be chosen.
Guinea pigs are very peaceful.
They do not get along well with mice, rats or hamsters. Dogs, cats and other bigger pets are also dangerous for our little darlings. As far as older animals are concerned sexual distinction is visible by their behaviour. Concerning younger animals sexual distinction is difficult to foretell as the organs are very small. If ever hold the animal in one hand and press carefully with your finger on its belly slightly above the sexual organs.