I see 5 core units there I believe?
Correct, unless you count the Flagellants (having a Warrior Priest lets one unit of them count as Core), then you've got 6.
Warrior priests are pretty good, especially for their enhancing spells but they stand little chance on their own against chaos warlords or vampires. Don't put him in your units of greatswords, they can handle themselves and should be used primarily as a means of holding down a strong unit so you can flank it with another (works better with flaggelants)
I know that other armies are usually tougher, stronger, and all around more buff than the Empire in general, including my Warrior Priest, which is why I gave him Van Horstmann's Speculum. It allows me to swap my A, S, T, WS and I with anyone he's dueling, and the way this would work would be to hopefully stick him with a unit that looks tender AND points-heavy to tempt enemy melee monsters into challenging him and getting the tables turned on them. The Sword of Justice (re-roll failed wounds) is there to help ensure the enemy just doesn't survive the duel.
So yes, he'd probably do better stuck in with the swordsmen and using Unbending Righteousness (prayer grants Immunity to Psychology) (which I chose over other State Troops because they get +1 WS, I, and a shield for 1 pt. more than either Halberdiers or Spearmen which I felt made them a better deal) now that I think about it. The only reason I suggested it was to make a Stubborn unit Immune to Psychology, which is a bit of a waste now that I think about it. Fearlessness for the win.
Make sure to select targets and whittle them down before destroying them. Empire doesn't do well on headlong charges without proper preparation. Use fastcavalry to disrupt enemy movement and harass smaller units.
Yeah the fast cavalry came highly recommended to me by a Fantasy player I know and Outriders, though they pack much more death per point, did not supply the sheer thrill of the ride-by shooting (fast cavalry that can't move and shoot? Boring) I was seeking, and what's the point of winning if it's not as much fun?
I find that splitting my hero choices between melee/casters doesn't work that well, it just tends to balance things out.
As I said, balance was one of the themes for this force. Is there really no good way for the two forces to work with each other? Like that Light spell that lowers enemy WS to 1, making them easier for my soldiers to smack around. Or is this type of coordination just a pipe dream?
Thanks for all the advice so far, if anyone has any more please share it. This has given me a good bit to think on.