Three sisters (beans, squash and corn) are the classic, since the beans climb the corn, and provide a good balance with the squash. 5-6 plants in a 2x1m might be a bit tight, be sure to feed well with nourishing compost, they thrive on it! Nettle tea is also good (soak nettles in water for a couple weeks, then use it to water the plants). Radishes are super fast in my garden, and squash slow, so I'd go for those, and some carrots. Carrots bring up more nutrients to the top soil, so that should benefit the squash as well. In general, I just try to interplant as much as possible, because that's the way they seem to like it. No momocrops for me!