Sorceress may use Fire, Shadow, Metal, Death or the specific DE Dark Lore. Whatever the Lore, they all have Power of Darkness, i.e. cast a spell to get more PD.
"Dark Elves" are spearmen.
Corsairs: Either Additional Handweapon or RHB. The pair of RHB is only for the champion.
Dark Riders may get a Crossbow (so they can double-move 18" and fire 24"). In that case, they cost more than 20 each IIRC.
Harpies are core but don't count for the 25%. They are skirmishers.
BG are limited to 20 per unit.
Cauldron: it is a warmachine. Its crew cannot challenge.
Beastmasters: they have no effect on movement phase. They cannot be targeted or killed, and they vanish when the hydra is killed (so no, there is never a monster reaction).
Also there is a DE magic item that allows the bearers unit to re-roll all missed hits ( unsure if in cc or not) once per game. I was on the receiving end of 20 xbows with Ench blades and this Magic item once. Needless to say my horde of halberd pretty much disintegrated.
Only for shooting. Usually given to BSB.