Backyard orchard culture

[mksmth]

I am by far an expert in this method of fruit growing but in my opinion it is a very intelligent way to maximize a small space and keep fruit trees in check. 
Dave wilson nursery and Tom spellman have some wonderful videos on how to prune and layout a backyard orchard. 
https://www.youtube.com/user/DaveWilsonTrees

I have 3 peaches in a row on 4' centers, 3 plum in a row on 4' centers and 3 apples on 8'. None will ever be more than about 7-7 1/2 feet tall. 
I hope to add 3-4 more peaches a couple plum and a few apples this year. May not happen that quick though.
this was taken last april. those are my citrus in the pots.   

[emegar]

I'll go ahead and be the one to state the obvious (or what should have been obvious to me, but wasn't): in order for dense planting (multiple trees in a single hole) to be successful, you want trees of similar vigor (perhaps same rootstock), and similar age/caliper.  I made the mistake of planting two plums and a peach together that were none of them the same age, and the results were what you'd imagine!

[funlul]

...in order for dense planting (multiple trees in a single hole) to be successful, you want trees of similar vigor (perhaps same rootstock), and similar age/caliper.

What about hedgerow planting? Is it OK if they are of different vigor / age?

I am highly interested in their idea as my yard is filling up very quickly but I crave over so many different fruits on this forum. But fruit trees only 30" apart just sounds extreme. However if successful, maybe I do not need to sacrifice the peach and apple tree currently thriving...