
"Ability drain", "HP damage" and "Level drain" indeed seems to cover most needs. Especially if HP damage would come in all the different varieties the engine allows (especially elemental/neg).
Even if ability drain and hp dmg are split to different feats between the abilities and damage types respectively, and of course between ranged and melee, it'd end up with about three dozen feats. Which seems still manageable, if the DCs are read off creature abilities as you say (or the DC could be a variable if simpler?), and damage dice number and sides are read off two int variables. Of course if it's easier to read damage type off a variable, too, then all for it. The point being that ability type seems more difficult a variable than just integers, builders need a lookup table on exact strings for each type probably.
Are you sure the feats are a better way to do this, than to use the "special abilities" tab of the critter blueprint?
*Is excited now*