if i was using dragon scale and 2 of trangs set, would the hydra from dragon scale gain damage from the fireball, ?
No since +certain skills don't synergise. Only hard points in the synergy will add damage, and paladins dont have any fire spells
i could be wrong
That might not be entirely true. I heard that it is possible to gain a bonus for the hydra in the form of Fire Mastery from the sorc. And pala's do have fire skills, but they don't synergise afaik. But as for your question: 2 trang parts do not add synergy bonuses.
what about a hellfire torch granting levels to hydra, would that stack with the hydra he's talking about?
Extra: iirc, there used to be a bug with Marrowwalks, some Necro boots that have lvl33 bone wall charges. Equipping those before 1.11b would give lvl33 synergies to all skills that had bone wall as a synergy (or is it bone prison?) instead of the maximum lvl20 hard points one can spend in a single skill.
(This would add loads of extra magic damage for your bonespirit/spear and add alot of damageabsorption for bone armor)