I did not get an answer as to the reason you are breeding. That being, I am assuming you are looking for a breeding setup that will produce a large number of births. If this response sounds harsh then it is because I misunderstand your reason for breeding.
My friend David has a herd of guinea pigs that he keeps to provide food for other creatures. I don't have access to the number of animals he removes from the herd. Judging from the number of births I would guesstimate that the natural doubling rate is between 6 and 9 months.
From what little I have read of people attempting to start a herd of guinea pigs, when you have only a limited number of guinea pigs, the herd may take over a year to double in size. You must understand that I have a very limited data to go on so this my be a false understanding on my part.
Definition:
Natural doubling rate - This is the rate by which a herd will double in size, unless a number of individuals are constantly removed from the herd. If for example the double rate is 5 months, then you have a herd of size X, 5 months later the herd size is 2X, 5 months later it is 4X, the 8X and so on.
Going on my understanding, it seems that a GP herd requires a sort of "critical mass" to reach to get to to shrink the doubling time below 12 months. It seems that if you start with two females and one or two males, you might eventually get a herd going, but it will be slow for the first year or two. In addition, if you remove one or two individuals, the herd's growth rate will will be slowed considerably.
My hypothesis is not that the GPs change biologically, but that the herd dynamic changes. It is theoretically possible to start a herd with one male and one female (provided you swap out some of the offspring with non-related GPs) and have the same doubling rate as a larger herd size. In practice, this does not seem to happen.
On the other hand if you have 6 or 8 adult female's and an least 3 adult males (NOTE1) the herd will double at a much higher rate than just 2F:1M. With rabbits many homesteaders will start with 2F:1M (a trio) and get a good production, however GPs don't seem to have a high production rate in that situation.
NOTE1: That sentience implied that a critical mass was 6F:3M, however this a just a guess biased on little date. If I could gather date with GP breeders I could make a firmer test my hypothesis.
My friend's heard has about 11 breeding females and 4 males that seem to occupy a position of dominance. Other males usually wind up getting scratched up trying to break into the dominate positions. Those males usually get removed from the herd rather than getting into a dominate position. About a year ago, David removed many males from the dominate position, some time later the herd's production seemed limited. There is a question on whether the seasonal heat or a disturbance in the herd was to blame for the slower reproduction rate. A few months later when the weather cooled and the younger males of the herd matured. Around the same time the herd returned to it's former production rate.
David's herd is a closed herd with no new genetics added in over 13 years of operation. As far as a genetic bottle-neck the herd size has never dropped below 40 individuals (including babies).
