We kept 70 - 80 wild guinea pigs in the back of our lot for perhaps four months. They free-ranged the pasture during the daytime and slept in sturdy cages at night. Every few weeks we would move the cages, till the manure into the ground, and plant a garden. Here are some insights:
1) The guinea pigs did a fantastic job of eating down about half an acre of pasture. They ate everything. That corner of the pasture was a mixture of hay and Spanish needles, which are both great for most small animals.
2) The guinea pigs tried to eat/gnaw anything and everything we grew, and a lot of extra work went in to protecting the gardens from the pigs. Meaning the pigs were useless at weeding the gardens.
3) No amount of eating down of the pasture provided ample nutrition for the pigs. Without a continuous supply of alfalfa pellets, the pigs became malnourished. We would have had to plant a large, continuously producing garden of cabbage in order to make the pigs sustainable.
4) The reality of how the alpha male pigs treated females was horrible. A lot of effort was necessary to protect pregnant or very recently pregnant females from the males.
5) Unless you can handle the guinea pigs from birth, they will make terrible pets.
6) If you want meat animals or pelts, you probably need to raise one of the larger breeds.
Despite the manure being a fantastic product for our gardens, we moved the pigs on to another group that would maintain them in a wild habitat and dropped that project.