Well, I was thinking that the larger volume a single shield generator has to cover, the weaker the shield is. Increasing the number of shield generators, or ramming more power though the generator offsets this, but there would be limits to these effects.
Also, by having main stream capships (Ie NOT the Lucifer) having weak shields would not seriously unbalance the game.
I'd also only let the Shivans have shielding over their capships, with only the most advanced, experimental GTVA craft having weak shielding.
Of course, all this is really boils down to personal preferance, since just about everyone knows how to handle tbl files.
Oh, and ATM, shields do not block other craft.