It's not the motor specs that matter, assuming you're happy with how it runs from your 11V source. The issue is how much current this FEZ thing needs at what voltage.
A linear regulator should work well enough as long as the motor doesn't temporarily make the 11V supply dip so low that the regulator can't maintain the output voltage anymore.
In theory, a regulator puts out a constant voltage as long as the input voltage is above some threshold. If you're trying to get 5V or 3.3V, then 11V is plenty in that regard. However, in reality regulators can only cope with a certain speed of input voltage change. Motors can cause large and sudden spikes, which can get onto the regulated output to some extent even if the regulator input stays above its minimum threshold.
Fortunately the solution to both problems is simple. Put a diode followed by electrolytic storage cap between the 11V supply and the regulator input. That will prevent negative going spikes from making it to the regulator input. The capacitor will hold up the regulator input voltage for the short duration of any spike.
In a generally noisy environment like what you describe, it's a good idea to put some high frequency filtering in front of the regulator. I don't know how much current this FEZ thing draws, but if its just a micrcontroller at let's say 200mA, then a ferrite bead or "chip inductor" followed by 10uF ceramic cap right accross the regulator input and ground terminals will do fine. This will attenuate the high frequencies that the regulator is not so good at dealing with.
Another point is that a linear regulator will be rather inefficient in this application. Even if this FEZ thing wants 5V, that's still 6V it will drop for about 45% efficiency. That by itself may not be a big deal since the power wasted as heat may be small compared to what the motors use. It's probably more a issue of dealing with the heat. Again, that depends on the specs of the FEZ. If it draws 50mA at 5V, then the regulator will only dissipate 300mW. Not a big deal for a TO-220 case in free air. If on the other hand it draws 400mA at 3.3V, then the regulator will dissipate over 3W, which needs to be specifically dealt with.
It might therefore be worth looking into a switcher. At 80% efficiency it would only dissipate 330mW with 400mA at 3.3V out.
You don't need a separate DC/DC converter. A motor controller that does Pulse Width Modulation (PWM) into an inductive (motor) load is essentially a buck-type non-isolated DC/DC converter. At an intermediate duty cycle, the voltage across the motor will be less than the battery voltage - try measuring it. Power is conserved, so
Vbatt * Ibatt = Vmotor * Imotor + [switching losses]
Note that if you limit the maximum duty cycle, you can safely use a higher battery voltage than the motor can withstand. This may allow you to add more energy storage (more batteries) to your project without being constrained by motor voltage.
Whether you decide to close the control loop with voltage (like a DC/DC converter), current (torque control), or go open loop (most small electronic speed controls) is up to you.
Best Answer
Let me start by saying that @JImDearden's answer is the correct one. You don't need a regulator for this motor. The answer I'm going to give assumes that you, for some reason, still need a regulator. While this doesn't exactly directly apply to this question, it brings up some things to consider that are worth knowing.
Don't Use A Linear Regulator:
Going from 7.4V to 6V will require a voltage drop of 1.4 volts. At six amps, that works out to be 8.4 watts. This means that your linear regulator will be outputting 8.4 watts of heat. That might not sound like a lot, but when it is concentrated into a small area it is enough to break things. You would certainly need a reasonable heat sink on this, just to get rid of the heat. But more importantly, that 8.4 watts of power will be wasted and since you are running on batteries this is not a good thing.
You Could Use A Switching Regulator, But:
A switching regulator can easily handle the 6 amp output that you require. However, you would want to carefully design it for maximum efficiency. If you don't take care in designing it, it would be about 80% efficient. With 36 watts of output, an 80% efficient switching regulator would waste 7.2 watts-- not significantly better than the linear regulator. With care, a switching regulator can be 90-95% efficient, and only waste 1.8 to 3.6 watts.
Switching regulators are also complex, and is beyond most hobbyist-level EE's. But buying a switcher module is an easy way to do it, without the difficult part of designing a proper PCB.
But PWM or Current Limiting is a good alternative:
Motors rarely care a lot about the voltage, it is total power (and thus, heat) that they care about. (There are some important caveats that I'll cover later.) It is possible, and common, to run motors at a higher voltage than what they are rated for. Two ways to achieve this is by using PWM or current limiting be within the rated power.
Let's say that you run the motor at roughly double the rated voltage. In that event, you can run the motor with a 50% duty cycle. Total power is essentially the same as if you ran it at the rated voltage but with a 100% duty cycle.
Alternatively, you can somewhat ignore voltage but run the motor at the rated current. Many motor driver chips can automatically measure and limit the current to the motor for just this purpose.
Caveats: Of course things are rarely that easy, and there are many factors to consider. I've just given a super quick overview. Here are some things to be aware of... Brushless DC motors (like muffin fans) have IC's in them that often can't handle voltages that are too high. Brushed-DC motors might wear out sooner due to increased arcing on the brushes at higher voltages. A voltage that is very high could cause the insulation to break down, so don't run a 6v motor off of 100 volts. Motors are highly inductive, which could help or hurt you when PWMing. And motor control is a complex subject, and you can make it as simple or as difficult as you want. But the closer to the edge that you push your motors, the more you have to pay attention to the details.