just a quick question, is damaged from effects assigned first ?

example, Zarathur's Flamers "action: sacrifice this unit to deal 2 damage to target none warlord unit at the same planet."

will this damage be assigned so that Zarathur, High Sorcerer will increase that damage by 1 ?

Yes. All "deal damage" effects follows the RRG's Dealing Damage process.

(Rules questions should be in the rules forum though.)