Not only is it possible to combine different trees in a food forest, it's highly encouraged. Large groups of like trees aren't forests, they're orchards or farms. One of the many advantages of food forests is that the incredibly diverse plant life slows the transmission of pest and disease between similar plants.
It was the peaches I was surprised to see on that list. I always think of them as a product of the deep south, forgetting how long people have been breeding them for different climates.
I'm in the middle of pecan's
native range, grow much of my garden directly under a pecan tree and mulch with pecan leaves so in my
experience that plant isn't horribly toxic to others.
Even if you have your heart set on black walnut, there are plants it plays nicely with. A fast search gives me an article from the Morton Arboretum that includes a list of plants sensitive to juglone and tolerant of them. The second list is much longer than the first. If you can plant
enough tolerant plants between your walnuts and other plants, even they are possible.
http://www.mortonarb.org/trees-plants/tree-and-plant-advice/horticulture-care/plants-tolerant-black-walnut-toxicity
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