I've been doing some spreadsheet number crunching on different shield combinations and I thought I would share some of my results. The optimum shield combination depends on three factors: the shield penetration strength of the opposing weapons, the expected DPS of the opposing weapons and your resource budget (cost, power, crew, weight, etc.).
One rule of thumb is that mixing shield types is almost always a bad idea. Resources spent on the more expensive shield types in a mixed combination are just being wasted. The one exception that I have found is that it is sometimes advantageous to mix Reflectives and Multiphasics depending on which resource you need to conserve most.
Against low penetration weapons (less than 24) either Reflectives or Fast Rechargers will reflect everything. Reflectives are cheaper. They are cheaper than Shield II's also.
Against mid penetration weapons (24, 25 and 26) such as Proton beams and Fed Fusion beams it's best to have at least one Reflective. Adding Fast Rechargers to a Reflective is a waste of resources. It's better to combine the Reflective with some mix of Reflectives and Multiphasics depending on which resource is most limiting. For example, if you are most limited by cost then you would use all Reflectives. If you need to conserve crew, you would want to use one Reflective and the rest Multiphasic.
Against high penetration weapons such as CL, missiles or Plasma, Reflectives are not good and Multiphasics are not disadvantaged by their low resistance levels. Against low to moderate DPS levels straight Fast Recharge works best. Against very high DPS/high Penetration weapons (think of a swarm of CL cruisers attacking) Multiphasics are best. Of course, in this situation all shields fail quickly, Multiphasics just fail a little less quickly.
Another thing, while increasing the number of shield modules you quickly run into diminishing returns. The total shield strengths maxes out at 9 or 10 modules and then starts to go down. However, increasing the number of shield modules continues to provide diminishingly better protection as high as I've tested it. You will run out of module slots before you run into a situation where more shield modules are worse. In practice though I don't think I would go above 4 shield modules.