Can these evergreens be trimmed to maintain an 8 foot height?
Emerald Green Arborvitae Smaragd
Certified GKH Gardening Expert
I don't usually recommend this. The trees are best left to their natural shape. There are some that grow into hedges, and some that can handle pruning more than others, but should you decide to prune it, you may not get very desirable effects. An alternative, if possible would be to replace them with a more suitable variety for the area.
This article will give you some general information on them: https://www.gardeningknowhow.com/ornamental/trees/arborvitae/growing-arborvitae-trees.htm