A food forest attempts to replicate the multi layer food producing elements the night be seen at the edge of the forest. Trees should be spaced according to their full grown sizes, and berries as well. Pollination should not be a problem since it's carried out by flying insects. Since you're planting different species of plants you don't have to worry about diseases that might affect a monoculture.
At the bottom of the food forest are your vegetables. You can either space traditionally ( see instructions on the seed packets), or grow closely together as in the French biointensive method. The later does give advantage of reduced watering due to the microclimate established, but does now readily allow pests to spread. However, proponents of this method says it results in strong plants that resist diseases.