"Nico's thinking, and therefore rules, about the shivan ship designs ( I'm gonna repeat myself, I know )":
1) When the Ancients talk about the shivans, they're telling us about black ships. When I look at the ingame ships, I see grey and red ships. And even more red if you take the inferno ships.
-> I'd make them almost black, with a few, but very bright red parts, and light grey parts for the weapons ( as explained a couple points down )
2) I hate the normal thrusters, they look too terran. The red bricks on the FS1 ships were perfect to break that feeling. Any shivan ship I'd do would have NO thruster at all, but red bricks instead ( check my mod site in my sig, the hydra is faulty, the harpy and the chimera, tho, are that way )
3) weapons: I guess missile racks can't be helped, but primaries, no, no. No gun barrels, bad, bad! Take the manticore concept pic, I'd always do that way, now. The large, bright emitter thinguy. Pb is, that should come with custom weapon effects: I don't picture the usual laser bolt, but more something looking like a shockwave emited from those things. I have a clear picture of that in my mind actually, and I think it looks cool.
4) organic looking ships: some concept pics show the shivan ships being clearly technological, very anime'ish ( especially FS1 concepts ), but the FS2 ones have undergone a severe organic cure ( see the ravana or the sathanas concept pics ). Blargh, that sucks. Organic goes to vasudans, it fits well their style. Shivans are like cold, feelingless killing machines. What does that makes you think about? Robots, yeah. Shivans ships must look like high technology incarnated.
5) The dreaded spikes. Modders have a thing for shivans ships and spikes. On a basis, shivans ships have 4 large claws, not more. And that's NOT a general trait: the mara has them, the sathanas have them. No FS1 shivan ships have that. Half the FS2 ships don't either. They usually have large, squarish fins.
Spikes are bad, plus spikes leads to think spider, organic ( see point above ).
6) Many shivan ships have that red triangle thinguy on the front ( mara ), or something alike. It's not mandatory ( FS1 fighters don't have those, but the capships do ), but given the evolution in FS2, I think it's a good thing to have those. That's an original shape that screams "shivan" to me.
7) assymetry: many shivans ships are assymetrical, but as many are not. In my books, both are fine.
8) shivan ships look mean, even crappy ones like the nephilim. That's mandatory

9) SCP wise, I think high spec on shivan ships should work fine ( spec on black ships -> high contrast, gives character to the ship, and allows to see details that people would claim being hidden by the very dark color scheme )
Voila, that's all, I don't ask anybody to follow that, I'm just stating what you have to expect from me if I were to do another shivan ship.