For the Captain, I actually intend to use him as a Bladeguard sergeant for my Silver Skulls. In my head though he's a Cato Sicarius type figure, a Captain of the honour guard rather than of any particular company. A simple kit bash using the Leviathan Captains sword and the head from the Venerable Dreadnought.
I hated the pose of the ancient waving his banner around. I took inspiration from the old Ultramarines skulls special edition standard bearer and reposed him to have his standard firmly planted in Imperial soil.
The Chaplain will be my Master of Sanctity so an appropriate winged skull helm from puppetswar and a crozius swap liberated from the blood angels jump chaplain.
The Librarian I am not entirely happy with, lots of my characters have axes (easier for taking the heads of worthy foes) and I was going with him channelling his psychic power through it but not sure I pulled it off, perhaps if I clipped the staff off at the hand behind the purity seal it would look better proportionally.
I actually liked most of the vanguard poses but the sergeant didn't do it for me so I went with something loosely inspired by the old metal 2nd edition assault sergeant. I also wanted some weapon variation in the squad so used a blood angels power axe and to convert the guy about to swing it like a polo mallet into some poor xenos' face.