If you're looking to plant zinnias and cosmos together, one important thing to keep in mind is that the two plants need to be of the same size. Zinnias have a tight, fleshy root system and can take up a lot of space, so don't try planting them next to each other. Cosmos, on the other hand, has a more spacious root system and can take up less space. You should also make sure that they are of the same color. If your zinnias are light green and your cosmos are dark green or blue, you'll need to change the plants around as they will not mix well.