Wren: Oh, the purple looked great! It just didn't belong on the base sprite. And I definitely prefer the longer-engine version, though King's suggestions do have some merit.
Brick: Yeah, that medium turret could go forwards a bit. ...I'm assuming it's medium? Actually, the mount point looks a bit smaller than either of the side ones; maybe it's for some sort of unique built-in weapon?
Alternatively (as in, instead of moving that turret forwards), try recessing the large mounts back into the hull some, with a bit of the flat armor peeled away to make room for machinery and cooling ports (like the machinery above the engines).
And speaking of engines: Offset some of them up or down a pixel or two. Right now the entire engine block is just a straight line at the back of the ship, and that doesn't look good. I'd probably shift the outermost engines up and the innermost engines down, perhaps with something in the middle also shifted up, aiming for a sort of a sawblade wedge shape.
Similarly, the sides are a bit too flat; yeah, they're not perfect straight lines, but it's easy to see how the whole hull could be represented by all of about seven vertices; that could use just a hair more complexity. I do like the engine block (aside from the above-mentioned issue), so maybe extend the forward wedge out a bit? Such an extension could also serve as a good spot for those small turrets; this looks like the sort of ship that'll run with a front shield, so who needs forward-facing PD anyway?