Since you only have a single variety of each, your only chance of cross pollination will come from a neighbor planting a different variety. You will have to let a squash and a cucumber ripen before the seeds are viable. The squash will get dry like a gourd, when dry just split and shake out the seeds. The cucumber will get big and yellow when ripe. Watermelon, Cantaloupe, squash, cucumber are all insect pollinated, so the possibility of an insect bring pollen from a neighbor is fairly high. On the other hand if you are in an isolated area, no worries.
