Well, "to the right of" means nothing. East? West? South? How far away? Etc.

But, probably no. Iris needs lots of sun, and hydrangeas will probably shade them.

Giving us the type of hydrangea is important here as is the type of iris. Panicled hydrangeas like a lot of sun and not a lot of water here in zone 5, and typical bearded iris would be happy in similar circumstances. The two other common kinds of hydrangeas prefer afternoon shade and/or more moisture to be happy. Siberian iris is tolerant of average to very wet growing conditions. Delphiniums range a lot in height from perhaps 18" to over my head, but all that I have grown like moist but well-drained soil with a lot of sun.

I always start by considering a plant's cultural needs and then look at the aesthetics. As Jamari's mentioned, whether you are aiming for extended bloomover spring and summer or everything blooming at once will effect your choices. Do you already own any of these plants?

Iris foliage is pretty ugly once post bloom, so consider that and placement...leaves get cut on the late fall shrub cleanup, but I would also plant them in a location where you don't need to see the foliage all summer...

I am thinking of planting tall bearded iris and endless summer hydrangea.

I haven't put anything into the ground yet. The irises I am looking at bloom around early summer.

If anyone has other suggestions for what to replace the irises with in this planting scheme I am open to that too.

Would the iris go nicer behind lavender perhaps?

Lavender and tall bearded iris like similar moisture and sun conditions.

Endless Summer is a big leaf hydrangea (Hydrangea macrophylla) and is one that prefers shade in the afternoon and more water than bearded iris.