If you consider that commercial orchards disk beneath the trees to eleminate competation for nutrents and moisture,the answer is clear. I think your plants will be better off with only mulch around them.

The standard permaculture advice would be to plant lots of different stuff in the understory, for different purposes.

The conventional thinking is that underplantings could steal nutrients or water from your fruit trees, and you end up with less yield. Another concern is possibly creating conditions for disease.

As for yield, with permaculture we go for multiple yields rather than a single yield. OK, maybe you get 3 or 4 less apples off a tree, but you get several pounds of grapes or blueberries, plus endless chives and mint and the bees are happier, keep your trees pollenated and yield honey. There is a balanced ecosystem, including habitat for birds, lizards, and beneficial insects so a single pest is unlikely to take over, and the whole system is more resilient.

Yarrow or comfrey are dynamic accumulators that can be chopped-and-dropped in place to fertilize your trees. Clover is a great nitrogen-fixing ground cover that can be likewise chopped and dropped, or left to grow and feed the bees. (also edible flowers).

I am thinking of growing some blackberry with nasty thorns up my nectarine to discourage raccoons & squirrels.

What about some variety of bean or pea? They are nitrogen fixers, so the nutrients they use will be somewhat offset by what they extract from the air (if you use the right inoculants), They are edible. They have flowers. They might like climbing the tree. And since most varieties are annuals, you can kill them off just by leaving the tree outside this winter if you decide they aren't working out.

This past spring I planted a North Star Cherry in a large container. Recenty i also thought about underplaning it with permaculture in mind so i planted medium red clover under the tree. These will fix nitrogen into the soil.

The clover is not very big yet but I attached a picture.

I have never done this before so it is just an experiment for me. I had extra clover seed and I thought the soil looked too naked. I thought about planting alpine strawberries, lowbush blueberry or herbs but I chose the clovers because they are good organic fertilizer. When I eventually cut down the fully grown clover it will drop the nitrogen nodes from its roots and release right onto the cherries roots.