Yes on covers for switches and outlets. Not only to prevent equine mischief but it will also keep them cleaner and that means more efficient and less likely to overheat due to dust contamination.
Re winter lights for broodmares, plan on using a timer for whatever type of light you favor for this. It will save you a boatload of money and effort.
Re switch placement, plan one switch that will turn on a light right by each entry door. Very often for “night check” all you have to do is walk through the barn. The only light you need for this is enough so that you see what’s what and you don’t trip over things in the barn aisle, skunks, snakes, etc. 
Bank the rest of your switches in a common location so you don’t have to walk over Hell’s Half Acre to find them and manipulate them.
If you plan on using fans, consider industrial grade ceiling fans. They have sealed motors, last a long time, don’t require plugs (meaning you don’t need as many outlets and that affect price), and can also be placed on timers or sensors.
Heat in Texas is not your friend (says this veteran of Houston and Corpus Christi :)). Ensure that you’ve done something to “vent” heat from your barn (fan, turbo vent, cupola, etc.).
Last, but far from least, take a trip to Best Buy or some equivalent and look at the myriad of devices and controls that you can now get and control from your phone. Cameras, properly set up, can eliminate the need for walking the barn aisle at night. Put enough low light so the the camera will function and now all you need do is use your phone or computer to do “night check,” foal watch, etc. What you will see at BB will likely be “consumer grade” and you’ll want “industrial grade” as you would for fans.
Good luck in your project.
G.
P.S. Don’t forget an outlet for your barn refrigerator!!! 
