Sorry for the delay, been out working .
I went and looked at your pictures, and i can see that the large beam above each stall will block any general lighting you put in the walkway.
Since your barn is so split up with the partitions and beams, i really dont know how to run this scenerio in my software program. it cant adjust for shadowing.
I think you will have to install a separate fixture inside each stall as others have said. They do make florescent fixtures with full vapor proof wraps, and i have installed these in wash bays and in stalls where people are worried about dust. My lights are 16 years old....using original lamps. Have never been cleaned, and put out awesome light.
http://www.homedepot.com/h_d1/N-5yc...keyword=florescent&storeId=10051#.URw8KlGxj7U
I have also installed these Jelly jar style incandescent in barns, and as long as you use a very powerful florescent spiral lamp i have never been able to get much light output from them. They do have new style of these that are LED based. However i have never installed these yet.
T8 and T5 lamps with electronic
ballast are good to -20F. My barn has the older style T12 High Output which will light to -40F. However, these styles are phasing out of production as i write this.
I too would use individual stall switching. I always wire barns in EMT conduit but never use 12/2 or 12/3 romex. Its better, cheaper, and all around easier to push #12 THHN stranded or solid wires in the conduits. If you have ever tried to pull 12/2 romex thru conduit, youll only do it once. it sucks.
1/2" emt can allow easily 3 circuits (7 - #12 wires) and 3/4" can allow more wires; however to be legal you have to derate current if you use more than 3 circuits in a conduit. I would run the main runs in 3/4" and all the drops (outlets, switches, light fixtures) in 1/2" conduit.
For the central barn, i personally would use Three (3) 8 foot T8 tandem fixtures (each light uses 4 - 4' long lamps. Use at minimum a 4,100K lamp for cool white lighting as this seems to be the best look in a barn. I have also used 5,000K and 6,500K lamps for REAL bright (almost bluish white) lights for clients that want it superbright. Spaced 2 feet from wall to 1st light. then 4 feet between fixtures.
Florescent does attract some bugs, but heck...so do horses. I swapped my exterior area light to a high pressure sodium light and this really helped cut down the bugs from the old metal halide i had there previously.