Sowing Radish at the same time as carrot and parsnip in the spring works well – I’ve done that one.
I planted some beetroot modules out underneath spring chard – they remained when I pulled the chard going to seed in early May and left them to mature until early July.
I’ve found that fruit trees certainly don’t get harmed by planting garlic, chives, leeks, clover underneath them – not done any science, but the harvests have been great since I planted them and get better – could just be we’ve had a couple of great summers for fruit though.
I routinely plant marigold in my tomato pots and I’ve done borage before too – definitely no harm there.
Berlicum carrots do very well after garlic – I harvest my garlic before the end of June here in NW London (sometimes early June if we have a warm, dry spring).