less of a hassle i think would be to get a bigger fridge off of craigslist and just put the fermenters inside. This would hold the temperatures more stable as well.
using coolant you're going to have a problem with overshoot in your temp control system, since the coolant is going to be at a lower temp than your target temp. the beer will also cool unevenly, getting real cold right next to the coils but staying warm in other regions. If you go this route, I would think that putting coils outside the fermenter vessel (but still inside your insulation) would allow the tank walls to even out your cooling.
commercial breweries general use a single glycol pump, but it works kinda like a power steering pump, where it is controlled to maintain line pressure that is released by the valves. How are you controlling the valves? Are you just going to "tune" them by hand and have the pump run constantly, or are they going to be triggered by individual thermostats in your fermenters? you have to account for the situtation where your controls close all the valves and the pump is still running. This might not be an issue, but if you're pump just keeps pushing against a closed valve, it could burn out. Also, the coolant would be more effective if you had a coil inside the fridge as well. Imagine the fermenters soaking up heat, then you're pumping that heat into the fridge, the fridge is then pumping it out into the room again. You need good heat transfer from the glycol to the air inside the fridge in order to maintin consistent cooling. Perhaps you could find a small car radiator that fits inside the fridge. That would be pretty awesome. Otherwise, I would just have the returning glycol run through a coil exposed to the air inside the fridge, before being dumped into the reservoir.
But still, I recommend getting a bigger fridge to make your life simpler, unless you're really good at building systems like this, which I'm not.