I would space that variety 3 feet apart. Amend the soil by adding one part compost, one part fresh topsoil, and one part sand (for drainage). If your soil is that old, I would remove it.
Water them as needed. Feel the soil to see if it is dry.
It would be better if you started at one end and continue on from there. In this way, you don't have to worry about fitting additional Arborvitae in between existing plants.
Arborvitae are pretty hardy. They are not fast growers, but are a good choice in narrow locations.