Are you looking for things to plant between rows or around individual trees? If the latter, any dwarf fruit tree that suits your climate. Dwarf stone fruits might be good, depending on the full size of your apple and pear trees. You could even look into extending your season with several varieties of stepover apples if space is limited (they make a lovely hedge).
Citrus are excellent either way if you don’t get too cold a winter as they can cope with some shade in summer and the deciduous trees will have lost their leaves when they need the sun in winter. There are several citrus that will tolerate a mild frost if they’re nursed a bit through the first couple of years. I have a cumquat, a Japanese mandarin, and a blood orange and a Meyer lemon in sheltered spots which are surviving light frosts with only mild protests, but they do require frost cloths once it gets below freezing.
I have hazelnuts and currants growing on the shady side of my fruit trees and they seem to be doing well. Both of these would otherwise suffer in harsh Aussie summers. They may be a better choice for a poly culture as neither are related to pomme or stone fruits.
You might also look at something like thornless brambleberries (thornless because they’d otherwise block your path to the fruit trees, a mistake I only made once!). I’d plant them closer in, though and use the tree as a trellis - my youngberry is currently doing an excellent job of protecting my lower crabapples from the birds, though they are taking longer than otherwise to ripen under it. Check how fast they reproduce though - one or two extra plants a year is fine, but you don’t want them to send runners voraciously across your entire garden. (My blackberry has a main stem and seems to only do a couple of extra bushes a year. I’m not sure if the variety as I moved it from a previous house).
