Your query is quite clear.
Advocates of sustainable agriculture (I am one!) believe that agriculture can only become sustainable if the practitioner studies and understands nature in general and his local environment in particular and then ensures full adherence to the fundamental guiding principles that are already in operation in nature.
The forest is considered to be an ideal example of a sustainable ecosystem. It survives for thousands of years and can go on and on unless human interference upsets the delicate balance that has been established within. Nobody goes and spreads expensive chemical fertilisers, growth hormones, pesticides, fungicides, weedicides etc in a forest. Nobody even cultivates the soil. Countless species of insects, microbes, fungi, rodents, birds, reptiles, animals coexist within the forest - but one has never heard of a forest dying off because of ‘pest attack’ or some kind of disease.
Agriculture was always based on observations of such ecosystems and for about ten thousand years, man has tried to mimic and harness nature for his benefit. Human beings ‘domesticated’ several varieties of grass (all our foodgrains etc) and that is how farming started.
One very important principle that is observed in the forest example is ‘Biodiversity’ - the coexistence of numerous species side by side. This can be considered to be the most important principle behind sustainability. There are several advantages that biodiversity brings. e.g.
Symbiotic, complementary relationships between different species of plants and animals have evolved over time. Trees make sugar available for fungi, while fungi play the key role of breaking down the decaying organic material on the forest floor and unlock nutrients for the trees. Earthworms feed on organic matter and ‘till’ the soil in a very beneficial manner and so on.
As foodchains get established, an all-round balance gets maintained automatically, No single species is allowed to overwhelm all the others, because the predators who are situated above it in the food chain will naturally grow correspondingly and keep its population under check. In mono cultures, this balance is lost. So if you have an orchard of only one type of fruit trees, or say a large farm of only one crop, it will always be vulnerable to the threat of a single type of insect proliferating beyond control and growing so strong that it destroys the entire crop. Then we are forced to respond with pesticides and the whole problem goes to the next plane!
Climatic conditions are beyond human control (though we seem to be doing a good job messing them up!) So if for example, there is excessive rain which is bad for the monoculture, the entire crop is at risk. However, in a diverse crop mix, one crop may suffer, but some others may actually benefit from the rain and compensate in some way for the losses. This has been experienced by farmers for ages.
Eventually, if the process of farming degenerates into a fight with A particular ‘pest’ (as as happened with all monocultures, sugarcane, alphonso, cotton etc) the risk grows greater. Because we are trying to ‘manage’ a certain species of insect with chemicals, we tend to lose sight of the amount and harmful side effects and characteristics of pesticides that we are using. The worst part is that the ‘pest’ in question adapts to the chemical and develops immunity in a few generations and you are back to square one.
These are some important arguments against the practice of monoculture.
Coming to your query - Even the second model with one acre sections of particular crops would be a technically monoculture. The solution perhaps would be to understand the threats to each crop and design the layout in such a way the all five acres would have all five species arranged in a mixed pattern that they would not compete with or dominate each other, and would actually be beneficial for each other in some way. Different species standing next to each other as close neighbors actually may work like ‘barriers’ for many pests with limited mobility, and ‘protect’ each other.
However, I think this will need detailed and deep study and sound knowledge of all the crops under consideration and perhaps even a ‘pilot’ model to test the theory.