Q: Can Captains of different Trek series steal each other’s Crew Members, or only crew from their own ships?

A: Any Captain can steal any Keeper with the Starfleet Officer/Crew Member icon, unless that Crew Member is already with their appropriately matched Captain. All Captains are commanding officers, so even though they are from different eras, if somehow they’re on the same mission (yeah, yeah, it’s time travel, or something), then any of them can order any crew member to join them – unless they’re trying to pull a Crew Member away from their “home Captain.”

And, of course, Captains cannot steal each other.

Regarding Worf’s situation: we see him in two different time periods in his life, and each version will stick with their current Captain. For those who are not familiar enough with the costumes from the two different series to tell them apart from their picture, they can also be distinguished by the title font, which will match between Keepers from the same deck. Also, Worf from TNG is a Lieutenant, while Worf from DS9 is a Commander.

The only Crew Member matching Captain Archer (in the Archer Expansion) is Dr. Phlox (in the Porthos Expansion), but he can still command others who are not already with the Captain they report to.