Few questions about spell damage

Hello there! Can some1 plz help me.

1. Is chaos dmg considered to be spell dmg? F.e. if i get some increased spell dmg (ISD) bonuses from some sources (there are plenty of them, passive nodes, gear, etc) - will my chaos dmg be affected by this ISD? Chas damage over time (DoT)? Do i need to cast it only from gem which has "spell" tag in case to apply ISD effect?

2. There are some passives with "% of spell dmg leeched as ES". So, if i use skill gem with "spell" tag which deals chaos dmg and chaos DoT and i have ISD bonuses - will i also leech more ES?

1 - Damage from spells is spell damage. Some spells can be chaos, but attacks can also do chaos damage.

Although some dots dont increase with spell damage, Chaos damage over time spells specifically says they do increase with ISD.

Essence drain, blight, contagion, all increase with spell damage

Yes, the gem should say Spell. (A few attacks specifically say they scale with ISD, but thats rare and used only on wands I think.)

2 - Damage over time unfortunately does not count as hits, and therefore will not leech any ES, even if you have spell leech on the tree. Essence drain has a built in effect that heals your health points for a percent of damage (but it does not count as leech).

There are spell leech nodes on the passive tree, but your skill generally needs to HIT, not be based on damage over time. (Some spells do both, and they will leech for the hit part if you have the right nodes on the passive tree.)