I'm sorry for asking if there's already an answer out there that I didn't find.

The text for the ability reads: During setup you may choose any number of death trooper deployment card as one open group

So I would want to ask if I choose use elite death troopers in that open group since it didn't specify. Another question its that do all of my DTs in that group get a power token through commence landing if I places one on it.

Yes.

• Elite Deployment cards share the same name as regular Deployment cards. When a figure is called out by name (for example, "Stormtrooper"), this refers to both regular and elite versions of this figure.

By taking one regular Death Trooper you get all 4 cards into the open groups for the "cost" of one.

The Death Trooper deployment cards are considered one group for choosing them as open groups only. Afterwards they are separate deployment cards and you handle them like any deployment card.

Commence Landing: No, you distribute 1 damage and 1 block token among the cards. (See Distribute from the ToL rules.) And when you deploy groups, you distribute power tokens from the deployment card among figures of the group.