Bad idea. The tomato will shade everything else, so they won't get the full sun they need. The garlic really should be planted in the fall and the onions in early spring. Tomatoes and peppers shouldn't be planted until night temperatures are reliably above 55-60. Even if you could get everything growing, there would be so much competition for food and root space, they would all be stunted and vulnerable to disease and pests. The tomato would do best all alone in a 20-gallon container.