Is the American Holly compatible with septic tanks and leach beds?

How deep would one have to go to get the roots to successfully transplant these in to the yard. a move of less that 200 feet.

It depends a lot on the specifics of your system. Most new septic systems are not prone to damage from smaller trees. Planting any type of tree directly over or very near a septic tank is not a good idea because of the need for access.

"How deep would one have to go..."

Here is a rough/generic rootball size guide for transplanting trees and shrubs. Soil type, tree species, and other factors are not taken into account, but it will give you an idea:

For trunk diameter below 1/2", multiply the diameter by 20 to get approximate rootball diameter.
For trunk diameter between 1/2" and 1", multiply the diameter by 18 to get approximate rootball diameter.
For trunk diameter between 1" and 1-1/2", multiply the diameter by 16 to get approximate rootball diameter.
For trunk diameter between 1-1/2" and 2-1/2", multiply the diameter by 14 to get approximate rootball diameter.
For trunk diameter between 2-1/2" and 4", multiply the diameter by 12 to get approximate rootball diameter.

Here are guidelines for rootball depth:

For a rootball with a diameter of 1', depth should be approximately 8".
For a rootball with a diameter of 2', depth should be approximately 1'.
For a rootball with a diameter of 3', depth should be approximately 15".
For a rootball with a diameter of 4', depth should be approximately 18".

dont get carried away with worrying about roots .. doing it properly.. including actual proper planting.. proper mulching and proper watering for 2 years.. is much more important than some giant root mass .. brandons reference guide is fine ...

ken