Stacking extra mods:
Mod 1 = 100% effect
Mod 2 = 87% effect
Mod 3 = 57% effect
Mod 4 = 28% effect
Mod 5 = 11% effect
Mod 6 = 3% effect

Rat damage types in detail here:

http://www.ogrank.com/content/view/698/59/

Roughly:

Damage type to protect against:
Angel - All
Amarr Navy - EM & Thermal
Blood - EM & Thermal
Caldary navy - Kinetic & Thermal
Gallente - Kinetic & Thermal
Guristas - Kinetic & Thermal
Khanid - EM & Thermal
Mercenary - EM & Thermal & Kin
Mordus - Kinetic & Thermal
Sansha - EM & Thermal
Serpentis - Kinetic & Thermal

Damage type to use against
Guristas - Kinetic
Serpentis - Thermal
Mordus - Thermal & Kinetic
Blood - EM & Thermal
Sansha - Thermal
Angel - Explosive
Mercenary - Thermal
Thukker - Thermal
The Seven - Thermal
Drones - EM & Thermal