"For a 5 gallon plant, how large should the planting hole be?  For the Arbequina variety, are there any other special instructions for planting and care?"

A good rule of thumb for planting holes is to make them one-and-a-half to two times the diameter of the container....

"Can you please tell me what does the three types of cold hardiness mean
-  low, medium and high in terms of actual temperatures the tree can
withstand.   Thanks"

It is hard to be that precise about cold hardiness in olives...
 

"I have a fairly large orchard planted with 2-3 year old trees.  There is a large amount of tiny suckers and low sprouting growth, I have read and been told to hold off on pruning till my trees are older.  But should I allow the tiny suckers and low level growth to continue.  Or if I should remove them should I treat the trunk with anything after clipping?"

You are correct to hold off pruning olive trees. Pruning olive trees too young will set them back and result in slower establishment....

"I would like to plant olives for oil but I am not sure what variety to pick. I prefer a fruity oil without too much of a bite."

The question of oil style is as much about harvest maturity as it is about variety. Any olive will produce a more bitter and pungent (peppery) oil if it is harvested on the green side....

"My olive trees look really peaked right now. They looked good last fall, but now the leaves on all of the trees are yellowish. They are 3 years old, mixed Italian varieties."

Yellowing leaves can be a sign of many things on olive trees—some are worrisome and some are not....