I realize that different varieties would require more or less spacing between trees. If you were permanently planting fifteen different varieties that are yet to be determined, what would be your minimum distance between each tree? Take in to consideration that space is at a premium.

4 feet, but keep them small, (4-5 feet high)
I saw a pic of Fence style planting (criss-cross shape) by one of our forum members, looks really beautiful, i think he planted them 3 foot apart...i might be mistaken though.

Mike,
5 gallon potted figs spaced 4 feet on center grew into each other last season, so I believe 10 feet on center for in ground would be my minimum distance for a bush pruned fig tree... the same as my dwarf apples and pears.

Is the question how much root space do they need, or how big will they get, or what is best spacing for productivity, or aesthetics?

I'm going with 15' OC in a triangle formation (just because of odd space I had allocated) with additional plans to interplant some small slow growers or pot culture in the center of the triangle. I believe I'll add, as I have been told this many times, that you can keep a fig tree at any size you want to.

My goal is to carry out a trial in ground for several varieties with limited space. After 3 years or so I would then keep the best third or so and space out the remaining trees so that they have more room to grow and be productive. So I am thinking of giving each a 5-6 diameter circle and maintaining height around 5 feet.

Check this out--with proper staking probably..

My Alma and Brown Turkey are exactly 12 feet oc for a 6 foot radius. They both reached about 8 feet last year before extensive pruning. They are far enough apart not to shade each other excessively and there is plenty of room to walk between them for now.  

Here is an interesting article about ones location versus row orientation for best light for a specific season:

http://allenpress.com/publications/pr/WEED58_2

Corn showing greater yield when planted North-South rows:

https://www.pioneer.com/home/site/mobile/agronomy/effect_row_direction_on_corn_yield/

If you don't have that option, put them where you can fit them, out of shade as much as possible for faster growth and more fruit. With a lot of different varieties, a fewer fruit from each tree may be a blessing if they still develop adequately.