About the skills, don’t forget that you need at least 1 point in Blood Bath to enable the FF attribute Increased Recovery. Tet Mamak La is also useful to organize the zombies (and very cheap, 1 point only).
Between Rod and Staff, Rod is usually better, but it depends really on the equipment. With a Rod you can use shields, aiming for more bonuses and survivability, but it is more expensive, (2 pieces instead of 1). Same thing for daggers. More equipment means more bonuses. For example:
With a Staff LV. 380, you can have:
-matk 5200
-INT +60
-SPR +48
-Add. Fire Damage +120
-Add. Damage vs. Insect +230
With a Rod and a Shield LV. 380, you can have:
-matk 4500
-INT +60
-SPR +48
-Add. Fire Damage +120
-Add. Damage vs. Insect +230
And also:
-pdef 2800
-INT +60
-SPR +48
-Fire Resistance +120
-Ice Resistance +200
As you can see, you have twice the bonuses. Same thing applies for dagger. That doesn’t mean staff is always worse (I myself use a Staff), but before choosing your equipment, check if the bonuses from a the Staff (combined with the matk) are more worthy than the ones in the Rod+Shield or Rod+Dagger.
About your question, when a equipment says Add. elemental damage X, it means that each hit (auto-attack and skills) will be added a fixed amount of damage of X. For multi-hit skills, each hit will be added this amount.