(Note: It doesn't benefit from “+X-Y Elemental Damage” affixes on weapons.)
+% elemental damage would only work on weapons with black damage but other than that there's no difference. it does work for the black damage just not the added poison damage part. http://us.battle.net/d3/en/item/crossbow/#type=common
hellion crossbow has ~500 regular damage so
manticore is about half regular damage and half added poison damage so if you use a 6% elemental damage soj you would only get 3% of the damage bonus. hope that helps.
for your second question no, ls can only be on weapons. but with gloom you don't really need it or if you prefer life on hit, it can spawn on jewelry and some legendary/set items too.
I tested this with a Wizard and Magic Missile (arcane dmg) combined with Cold-Blooded and an axe with cold damage. Damage went up about 20%, as stated in the skill description. It's well known this isn't supposed to be happening, but it seems to be anyways. I'm going to do some more tests to see what's going on.