Don,
I planted one Thuja Green Giant last spring. Had no perceptible growth last year, looks like maybe a couple of inches so far this year. I'm in Z8 Tx. with clay soil and water weekly. My Eastern Red Cedars grew between 18-30" under the same treatment last year and have already grown 6-8" or more this year. I have not seen my TGG even achieve acceptable slow growth(1')in Z8, but may have other limiting factors you may not have.
For size of trees, I get more reliable results with 3-4' trees than say 6-8' in general.

It is true that the TGG might not begin to grow much until after the first spring and fall they are planted. A lot will depend on the planting bed's drainage rate, how well it retains moisture without remaing soggy, how much quality humus is in the planting bed's soil, and how much protection the area gets from moisture evaporating winter sun and wind.

If you start out with 3-4 foot tall potted trees, the area should still look nice even before they have grown much; especially if you have enough room to plant two staggered rows of either the Steeplechase, or Green Giant. You could even consider planting a back row of T. Green Giants and a staggered front row of Steeplechase.

Just one question. The eventual height of TGG will be 60', with 'Steepelchase' about the same, with a projected height of 30' in 30 years. Is this the height you want, or should you look for something that will mature at about 20', but might be a bit slower growing, and will create less of a Great Wall of China effect?

Of course, since you may not still be liiving there in 30 years, you can always plant them anyway, and let the then owner of the house deal with it.