Torque of motor vs operation as generator
So, is there any relation between motor torque and/or wattage with its generator torque? 


Generally yes.  To a pretty good first approximation, the torque that a DC motor gives for a given current is equal to the torque that same machine will need to absorb to deliver the same current as a generator.  The biggest difference will be the machine's friction torque, which is always a loss either way.


  Clearly, for a smallish turbine that I'm aiming for, I'm looking for low startup torque.


For an ideal DC machine, as long as there is no current, there is no torque.  So in that sense "startup torque" is entirely under your control, or at least under the control of the electronics you use.

Typically what's not ideal about a DC machine is the friction torque and cogging torque.  These are often not well specified in datasheets (and aren't specified at all if you're buying a motor for hobbyist or consumer use, such as an RC plane motor).


  Or more in general, how does one decide which motor is best for a given set of blades?


You find the speed and torque at which the blades are most efficient, and you find a machine that will absorb that torque at that speed -- or you choose to drive the motor through a set of gears (probably step-up) to spin the motor at a different speed, while absorbing the same power.

Then you figure out the speeds and whatnot for when the wind conditions aren't ideal (most especially you check what happens if the turbine goes overspeed), and you deal with those.