How much sun does this bed get? It just seems slightly strange to me to consider combining hostas - shade lovers -with full sun plants like coreopsis and dianthus. And it is difficult to get more specific about placement without knowing what kind of hosta and how big it is. Generally, you want the larger plants in back with smaller/lower gowing plants in the foreground but that's not necessarily cast in stone, depending on how the planting area is viewed.

The experts on the Hosta Forum will tell you that most any hosta can be grown in full sun, with enough watering. Do you know what hosta you have? Some can be 8 feet wide or more at maturity. You may want to divide yours if it is taking over. Or chop a hunk of it and transplant the hunk to another part of your garden. Or give the division to a friend if you have no room.

Here is a link that might be useful: Hosta Forum