First, when planting next to walkways and foundations, one must take into account the lime that leaches out of masonary and tends to sweeten the soil. Azaleas need an acidic soil and this lime neutralizes the acidity in the soil. This can be compensated for with powdered sulfur that is worked into the soil before they are planted and spread on the surface after they are planted.

You don't want to jam too many plants into an area. They need room to get sun and air. As a ballpark guess, I would guess that your azalea will be about a 18" to 2' plant and may grow to twice that size eventually. Hence they should be about 2' to 3' apart. The hole you use to plant should be about twice the size of the current root ball. You don't want the planting holes to touch.

Here is a link that might be useful: How to grow azaleas.

Hmmmm. Azaleas come in both evergreen and deciduous varieties, in varied sizes when mature. It would help to know which you have :) And the approx depth of the bed from sidewalk to foundation.

You also don't indicate your zone or climate. While azaleas do well in full sun here, and the deciduous types in full sun many places, that's not true of every gardening climate. I'm guessing your site is sunny since you have roses and sedum there now?

Comparatively slow and small evergreen azaleas look irritated with other kinds of plants jammed up against them or flopping on them. Sections of azalea will soon thin out markedly, even die away if shaded much. They do like the intimate company of others of their own kind, with small leaves their fine texture will not look in proportion unless there is a number of them planted all together in a drift or other group arrangement either.

Plant it where you can include at least several others in a close grouping. Let them trap and hold fallen leaves, create their own mulch. Rhododendrons and azaleas need cool root zones.