"Soul Absorption"'s effect is based on the number of cards, not on each "lump sum" of cards. So if 3 DARK monsters are removed from the Graveyard to Special Summon "Dark Necrofear", the controller of "Soul Absorption" gains 1500 Life Points.
However, you receive the Life Points from "Soul Absorption" as a "lump sum", so in the case of "Dark Necrofear" you gain 1500 Life Points but your "Fire Princess" would only do 500 points of damage.
"Soul Absorption" is a "trigger-like" Continuous Spell Card, not a "continuous-like" Continuous Spell Card, so activate its effect after the current chain resolves that includes removing cards from play.
The effect of "Soul Absorption" can, itself, be chained to after it activates.
If "Soul Absorption"'s effect activates in response to the Special Summon of a monster like "Dark Necrofear" or "Inferno", Soul Absorption's effect activates after the monster is Special Summoned.
For "Chaosrider Gustapth", activate "Soul Absorption"'s effect after "Chaosrider Gustaph"'s effect resolves.
Your "Soul Absorption"'s effect activates even when your opponent's cards are removed.
Spell|
Continuous