I daresay you that, as far as an empty garden may look barren, it's always wise, in rotation, keeping parts of soil covered with flowers and parts with just "english cropped grass". Is like with every other cultivation: earth enjoys a period of rest every now then.
However, you'd be amazed to know that what you found by accident, someone else found out by deliberate research ( http://www.ehow.com/info_8067329_can-pet... ), and actually the couple Petunias plus Marigolds is well known as a simple way to have a long lasting flower bed with an annual bloom, thus avoiding the strain of perennials.
However, here's a list of early bloomers:
However, Tulips come in many variety, late and early bloom. And can be easily "Layered" (that means, planted in the same bed) with no ill effect. However, this way you may seem to lose the variety you achieved with the couple Petunia and Marigold.
Also, daffodils are nice early bloomers, and they also look nice with the late blooming tulips and hyacints.
I personally dislike allium, and I must confess you I've never had a garden in ages, when I was much younger and spent the warm months in a nice vacation home with a simple english garden and a topiary, but I think you could also use this guide useful