These guys are pretty low maintenance as well, but again husbandry is the key to their happiness.
Guinea Pigs originated in the jungles of South America where they were, and still are, a staple food source. They are very social, and do best in groups or pairs (not as solitary pets). They are quite vocal, with sounds ranging from chutting and purring, to chirrups, squeaks and squeals and shrieks when alarmed. If mixed sex groups are kept, it is recommended the males are desexed. They do establish a male-dominated heirarchy and the addition of a new male will cause an upset from just nipping and squeaking at each other to 'barbering' the hair of another pig (although this can happen with boredom and stress too). These little guys do NOT appreciate heat.... they prefer temperate climates and can easily overheat if ambient temperatures reach about 30C.
They can be startled easily, and like to hide, so need to be provided with tubes, boxes and the like that they can bolt into if startled. Enclosures should be large enough for decent foraging (at least 1m square), and have hides all over as Guinea Pigs will tend to keep to the edges if the enclosure is open. Wire is the most sensible barrier as they will chew through wood and plastic. Floor substrates should be natural dirt and grass, or wood shavings or newspaper litter which is regularly changed to maintain hygiene and remove waste.
Guinea Pig mouths are much the same as rabbits, in that their teeth constantly grow and need constant wearing to prevent overgrowth and eventual inability to eat. Commercial COMPLETE pellets are preferred to the grain-based ones and beware that an abrupt change of diet may cause anorexia, even down to the water source. Again (like rabbits) grass or grass hay is a good source of fibre and silica for dental health.
Guineas are unable to synthesise their own vitamin C and this is where diet is really important. Dandelions and leafy green veg can be added to supplement Vit C and provide environmental enrichment.
Sows are able to breed at 2-3mths and are in fact encouraged to before 6mths as at this time their pelvis hardens and will not widen for births resulting in labour difficulties and death. Prior to this it is malleable and a pregnancy serves to permanently mould it into a proper birth canal. Multiple sows and young can be housed together and they will jointly raise the young and 'cross-suckle'.