Yes, you can mix different types of plants together if you prune them as needed to manage growth. Herbs, Flowers and Salads are easiest to mix and match together. Tomatoes/Peppers mixed with any other plants will provide more of a challenge, and in some cases should not be mixed.
Herbs and Flowers offer the most variety and the greatest range of heights. Prune taller, faster-growing plants from the top to keep them an inch or two below the grow lights as they mature. This ensures that shorter, slower growing plants get the light they need to thrive. If you raise the grow light hood too soon just to accommodate the tall plants, the short plants will not receive enough light and could die off. Also trim leaves and branches that encroach on the space of others to open up light to the entire garden. Some plants will never grow as tall or as stout as others, so just be sure they are all getting enough light to remain hearty and healthy. The rule of thumb is to prune no more than 1/3 of any plant at once, so just a snip here and there should do the trick.
Salads/Lettuces usually sprout in about the same time frame and grow to about the same height, making it easy to grow different varieties together. They will require little or no pruning from the top, but individual leaves may need to be trimmed off at the base as they begin to crowd together.
Tomatoes and Peppers are the most challenging to grow with other plants because they spread out substantially as they grow (thus the packaging of fewer seeded pods in some tomato/pepper kits). Other plants may not have enough space to grow once the tomatoes and peppers begin to mature. Also some of the tomato varieties that we offer will reach 24" in height and are made for tall AeroGardens models only. These varieties should not be grown with other plants as the light hood would have to be raised to accommodate their height and the other smaller plants would not receive sufficient light. Sweet Bell Peppers also fall into the 24" category, so they should not be growth with other shorter plants.