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If you want to get properly balls deep into it then I can highly recommend you this 2 volume set of books. Quite expensive though but the information they contain is unrivalled.
How densely planted is the orchard? I know fuck all about it really but don't they typically plant them so the crowns are touching? Which means the canopy cover will be 90-100%. You might want to carefully select a few to fell to create more space for other plantings and open up the light available in places. Best way is to probably draw up a list of species you will want to plant and go from there. Working out what requirements they need in terms of light/water/soil etc. and finding a niche for them in the existing ecosystem.
If you wanted a cheaper (and more picture heavy) resource then Creating a Forest Garden by Martin Crawford is another excellent book. It's a shorter read and I'm using it more as a reference for species selection and their uses like nitrogen fixing ground cover, perennial climbing vines etc.
