Running a rear is more of a PITA because every time you change gearing or your chain, you have to match your rim up with your brake pads. This can lead to you pulling out your hair in frustration and will offer you less gear ratios. As to using a "rear" caliper on the front, many modern calipers come with a certain amount of toe in already built into the brake caliper. Thus, running a rear brake on the front may lead to inefficient and noisy braking. As well, a rear brake caliper uses a different attachment bolt that may well be too short to go all the way through your forks.
You have a rear brake on a fix. It's called your pedals. And the only reason not to use a rear caliper on a fork is the length of the center bolt. Obviously, if your front caliper is damaged or for some reason unusable, you can swap out the center bolts.
Yeah, I'm a car guy so I realize that the front brake is far more efficient in stopping force, but I wasn't sure if there was a specific reason for one over the other.
I actually only have a rear caliper, but I'm pretty sure i'll be able to adjust it to function on the front no problem. That whole adjusting gear ratio's is some good logic too, since this is my first setup I'm sure I'll try to be switching cogs initially.
But yeah, I'm gonna be taking the freewheel off of my wheel next weekend or so and i'm pretty sure thats called a "suicide hub" for a reason so it'll be just for an e-brake anywho.