Roger that,

Would there be any point in sending in a mixed fleet e.g. including light fighters? Would they not matter at all or would they simple get destroyed before the shield itself - hypothetically?

Trying to get to grips with the battlemechanics - soo much reading and plundering to do and so little time

The variables for shield destruction are too many to quantify with a simple scenario. The primary concern is the combatant's Weapons/Shields/Armor techs (WSA). Someone with little WSA (say 6/6/6) would be toast against a very small fleet of greater WSA (say 20/20/20).

I suggest that you use the Battle Simulator in the menu (or one of those offered on-line) to determine the possible outcomes as there is no "rule of thumb".

I don't consider shields to be a deterrent in any situation except to guard against casual cargo farming.