Do you mean espalier and cordon training? 18" would be right for cordons. 6ft might be just about enough for espaliers on dwarfing rootstock. I'm not clear on how a fruit tree hedge would work otherwise, unless you're talking about soft fruit bushes. What fruit are you envisaging? And a hedge row by definition would be smaller trees pretty much touching each other. Large trees would just be a row of trees, not a hedge.

No, I don't mean either. I mean Dave Wilson's backyard orchard culture style hedge rows:

http://www.davewilson.com/home-gardens/backyard-orchard-culture/photos/hedgerow

I would suggest not using multi-fruit trees. Typically one branch/variety will dominate and the other branches will hardly grow at all. You should use only fruit trees grafted onto small-tree rootstock; they will get plenty tall enough to accomplish your goal. Your idea: not sure what will happen when they want to grow across each other.... most fruit trees need air circulating so the middle branches are thinned-out and the branches go outward in a vase shape.

Yes, I should have posted that link in my original post.

I am mostly planting stone fruit like apricots, peaches, and nectarines.
I guess I am really just trying to figure out if it would be better to plant 20 trees at 4' apart and prune them heavily to keep them really small or 10 trees at 8' apart and let them get larger and graft on all the varieties I want. Dave Wilson is in the business of selling trees, so of course they would advocate for the former.

Are they going along a fence?

I concur with planting single variety trees. Apple trees lend themselves well to espalier, and they bear fruit on the same spurs. The branches can be extended out quite far if you are gong to espalier. The main reason for planting trees close is if you are going to keep them short and you have limited space. Without support, there is a limit to how long you can let the branches get without risking breakage. You also have to prune every year.

The trend in fruit tree growing, even in orchards is to keep the trees low enough to pick fruit without a ladder. If you are going for variety, choose fruit that ripens at different times of year. Dave Wilson has a good chart. Fruit Harvest Chart

Also, be sure to pick varieties that have appropriate chill hours for your location and also consider the need for pollinators for some varieties.

Thanks, I will stick with the single varieties then.

Yes, they are going by a fence. How close I plant them to the fence I will need to decide. I need to leave room for veggies, so I can't go too far from the fence. I do plan on keeping them short... I love being out in the garden, so happy to prune away. I have about a 80' of property line I can plant. I don't really want to mess with espalier... the fence is too short anyway (and I don't really like apples that much) . Right now I am thinking about 5' or 6' apart and about 5' away from the fence for the stone fruit.

Anyway, I have read all about chill hours and pollinators... I have most of the varieties decided although struggling to pick a second apricot or aprium in addition to the Blenheim I want to plant (apricots are my favorite and they all sound good). Also all the figs and pomegranates sound yummy. Dave Wilsons site is dangerous... so many delicious sounding fruit. It is hard to pick just a few!

I looked at the pictures and GG is right. That's not what I had envisaged by the term 'hedge'. They're just rows of trees to me. He's using the word in his own way. And those rows which could be called a hedge are planted at an angle and grow into each other. They're pretty much cordons.

I agree with the others that the distance depends on the varieties and not to do family trees.

You may have vermin problems planting along the fence. Squirrels and rats love to use the fence as a highway of sorts. during the dry season of California they really attack the fruit because they need water.

Being near a fence might make it easier for critters to munch on fruit without much effort, but if they want fruit, they'll scamper down the fence and up the trunk in seconds no matter where the tree is.

Yeah... there will be critter problems no matter where I plant anything... it is the cost of growing anything tasty.

If you are okay with adop a barn cat at a local shelter it will help tremendously