Well the scale thing is covered with the size rule. Size influences the Structure Points you have, i.e. the larger the ship the more it can take. So its much harder to destroy a galaxy class than a fighter.
If you refer to the ships in the Dominion War Sourcebook, I have to say that I have reduced each ships shields, because they were in no comparison to the shields of ships n the ordinary books, but that is personal taste.
And yes each shp has two shields, one aft and one front. For each you have to pay the energy ( as said in the rules ). that means a Galaxy Class has shields 60 ( 80 ). So you have to spend 120 power to bring up shields on both sides at a rate of 60. Starbases have four shields so you have to spend for four parts. However I tend to cover shields with one 'payment' of power because there are ships you can't even afford to get their shields fully up ( see the Saber Class with 130 power ) which I think is nonsense, why shouzld Starfleet built them that way?
So its up to you I think.
By the way your English is good.
We came in peace, for all mankind - Apollo 11