Greetings Lewan:
I don't think you need to worry about the southern pine beetle seeing that Thuja Green Giant is not in the Genus "Pinus".
Based on what I've read, Thuja Green Giant is adapted to a wide range of soil types, including heavier textured soils (i.e., clay).
More on Thuja Green Giant: http://www.usna.usda.gov/Newintro/grgiant.pdf