Thank you both for your comments and suggestions. I played a game this past weekend with the models as-is (Doomsday Arks and all) against IG, and I LOVED how the army performed! My other armies are Orks and Black Templar, so I'm used to up close armies that move quickly, so it was an interesting change for me.
Thoughts on the army's parts:
Warriors - Absolutely fantastic. Lost 13 in the first round of shooting from a squad of 20, and over half came back through lucky rolling! The gauss flayers are great for taking out infantry and vehicles when fired in large groups. The warriors were responsible for glancing two Russes and a Valkyrie to death in four turns.
Szeras - Meh. He did his job in combat, causing a bit of a stir. He gave one unit of warriors a strength bonus, and they never made it to combat. I could have used the BS or T bonus instead, but I'll take it.
Stalker - Scary thing, at least for my opponent. I think it is a mental thing - there is a giant, spider/crab like walker on the field, so it draws fire. It only lost a single hull-point and dropped a Valkyrie with its Melta Ray (can't remember the actual name...). Also scary in combat.
Scarabs - Fun little guys! I ran them up the sides of the board behind squads of warriors, then used them against infantry squads. They pack a HUGE punch just by providing so many dice, and IG infantry can't really stand up to them. I never got a chance to assault any vehicles with them.
C'Tan Shard - An expensive scary unit that does its job by absorbing as much fire as possible. It didn't die until the last round of combat, blowing up and taking out a couple of other models. I used the assault 8 power and entropic strike, I believe (it was something to do with combat, yet it never really mattered). Doubt I'd take one in a tournament, but its great for friendly games just because it draws fire away from everyone else.
Doomsday Arks - Sat still in the back and fired at his 1 Griffon/2 Medusa combo unit until they died, then fired into small blobs of infantry for fun. One was immobilized facing forward on turn one, but luckily it was looking directly at his Medusa! Not my favorite units, but they did a fair amount of damage.
I'd like to try Spyders, Wraiths, and Ghost Arks when I play again, proxying them to see what I like and dislike. The jetbikes and chariots look fun, so I might have to try them as well. All-in-all, I'm loving my new army and glad it plays differently than my beloved greenskins and overzealous Templars.