Wow, I'm surprised you guys didn't get the answers by now, if not by trial and error.

Yes, they [goblin shaman] give it only to existing goblin stacks (not orc). They do not give to themselves, and they DO give you above your leadership limit, as they start filling out your reserve section.

e.g.
I had 3 goblin shaman, 5 goblins, 3 catapults, and 2 other non-goblin units.

I win a battle.

I randomly get +3 goblins which end up in my reserve troop area.

In another battle, I get +2 catapults which end up in my reserve troop area.

To be frank, it's a great concept, until you want to hold other reserve troops and you end up losing slots to these other units.

It can definitely help out early on (which is why I've been saying Crossworlds seems to be FAR smoother, especially for impossible mode games).

I don't use orcs. My first walkthrough will be with a human army (well those, and Witch Hunter/Royal Griffins), so no, I do not know from trial and error.

And Orc Chieftains do give Orc troops, ckdamascus. Have not used them, but ZagZag (Rusty Anchor) has them so I checked.