I have been designing plans to train some different small fruit trees and dwarf holiday avocados horizontally along my fence in the following pattern:

(7' fruit)(4'avo)(7' fruit)(4' avo)(7' fruit) and so on for the length of the fence...

Spacing between trees will therefore be 5.5 feet. I know this is less than recommended, but I have trained small fruit trees even closer together before and never had any problems -don't know if I got away with something there, please let me know if you have any insight, tips or warnings. Visually speaking, my hope is that having every other tree be a small holiday avocado will lend some structure for the eye so it doesn't just look like a bunch of different fruit trees growing in different patterns and rates up against a boxy fence. Yet, I want a diversity of fruit so that limits what I can do as far as visual appeal. Any insight on the visual aspect is also appreciated.

Now, my main question:

On the other side of my fence, my neighbor has an ornamental pear tree, about 12' high and about 4' from our mutual fence which I am going to call the "subject tree". This means I will have to plant one holiday avocado and one other fruit tree about 6-7' away from the "subject tree." To minimize the chance I will end up with a dead or unhealthy couple of trees in my otherwise organized row, which fruit tree should I choose to plant nearest the "subject tree'?

a) the most vigorous fruit tree, because it will have the best likelihood of holding its own against the roots of the "subject tree"?

b) the smallest fruit tree, because its root structure is smallest and will have the least likelihood of conflicting with the roots of the "subject tree"?

c) Some other way of picking the best tree?

By the way, it is possible but not certain the "subject tree" will be removed within 5 years. Thanks for any insight you can provide