Fans
 

Any pros or cons using a 5 volt 40mm fan off the receiver battery as opposed to 12 volt 40mm fan off main batteries?

Generally, the closer the fans working voltage to the supply voltage, the better. If you are running 12 cells for your main batteries, that is at least 14.4v (as high as 16v on a full charge). A 12v fan will fun faster and may wear out sooner. Any more cells just makes it worse. Running a 5v fan on a 6v pack is the same idea. A 5 cell pack can run as high as 7v on a full charge. They will run, just a bit fast. If you are running a UBEC, that only puts out 6v, so it will be closer no matter the charge level of the batteries.

I'm running 16 cells so 20 volts or so....how long you think a 12v fan would last? :)

Hmmm, let's see, overvolted by 8v on a 12v fan. It's hard to say, but wow will it spin fast! :) If you are just using one fan, you can use a 12v regulator from RadioShack. Simple three terminal hookup (input, output, common). Since a fan doesn't pull much current, heat should be minimal. Figure out the regulator dissipation by using 8v*fan_current. If the dissipation is greater than 1/2 watt, also use a small heatsink. Note: the little tab is internally connected to one of the leads (output I believe).