"Win the war before your enemy realises that you are theirs. Knowledge is power, so remove their strength, not by rendering them blind, deaf or dumb, no. That is the way of the VIII Legion, not the XX make them weak by letting them assume all is well and good,that they are safe in their homes and chattels, when the very foundations of their fortresses and bastions have been extinguished. Only then, when victory is a foregone conclusion, do you make your intentions known"
- Attributed to Alpharius, Primarch, XX Legiones Astartes.
Planetary compliance procedure [Infiltration Protocol] Phase One: Infiltration

Silence their information gatherers. Take their place if needs be.
"Leave them looking, leave them guessing how you appeared behind the rear guard. Break their defences, then begin the slaughter."
- Attributed to Alpharius, Primarch, XX Legiones Astartes.
I made up another few Alphas, giving me the beginnings of a Legion Veteran squad - Heavy Bolter, 3 boltguns, and Legion Sergeant with combi flamer (sniper flamer, anyone?), power dagger, and powerfist.
Calth is an interesting box, and a nice start to a Legion. So far, I'm thining:
Skorr, Sabateur, legion veteran Rite, 2x veteran tacticals, seekers/headhunters, Contemptor.