Von Schill transitions to a Viktoria crew pretty easy since both masters generally play "Good Models".crew with very little synergy among their pieces. The parts of each crew box also play well with each other. Ronin and Friekorpsmen are fairly interchangeable depending on what you need, Taelor is a solid heavy hitter for the faction, and the Librarian and Trapper are pretty staple model. Each of the large base Friekorp models are also very good depending on what you need to achieve.
Digging into Leveticus usually involves taking a dip into either Guild, Arcanist, and/or Ressers depending how you want to play him. The Pariah upgrades give him some powerful hiring options depending on what you need to do. You don't need to go buy every piece he can hire, but they'll be a few things only he'd be able to run until you dip into other factions.
If you want to stay "Guns for Hire" besides the Viks, Tara can do that theme pretty well. While she wants to play a few void piece, and picking up some Death Marshals isn't a bad call, she can run the standard "Good Models".crew pretty well some Fast and Burry tricks.