Polycount limits as such are non-existant. Build models with as much detail as you like, but in a way so that the small, poly-hungry stuff is easy to remove. The theory is that while we can always easily scale down detail to fit the machines it runs on, we can't so easily add detail that isn't already there after the ship is completed.
A rough in-game polycount guide (and I must stress that this is only a guide) for now would be:
Fighters: 3k-6k polys. (My Loki is a tiny bit overdetailed and weighs in at 7392 polys for example.)
Bombers: About the same - maybe 3k-7k.
Cruisers: Anywhere around 3k-10k.
Corvettes: Anywhere around 5k-12k.
Destroyers: Anywhere around 6k-25k. (Assuming detail boxes and that you're NOT using texture tiling, or else it would be around 3k-10k max. Texture tiling sucks.)
Juggernoughts: HUGE range here - about 10k-40k (with no texture tiling, and good use of detail boxes. 4k-10k otherwise.)
Making LODs is very easy. If Blender has an excellent poly-reducer tool then I'm sure Maya does too, so find and use that. Typically using it will invovle making a duplicate of the model and moving it aside/above the main one, applying the poly reducer repeatedly until approximately the desired polycount is reached, duplicating and moving again and repeating the process until the ship has 3-4 LODs. The key to good lodding is quite simply to have increasingly large polycount drops between LODs.
The first LOD (LOD1) is the one always seen in the target box on the HUD, so it needs to be the most efficient of the lot. I try to pick out the small details that will be most obvious when the ship is approximately that size and poly reduce until just before those features are seriously compromised. By the second LOD you should just be trying to keep the overall profile and colours, so you can usually take this one down to about 500 polys for ANY ship. LOD 4 if there is one should basically just be a box with the same colour but I think this one is a bit unnessecary though.
As for debris, my exact process is constantly changing, but a rough outline of it would be:
1) Take LOD1 (DO NOT TAKE LOD0!!!! It's a massive waste!) and duplicate it.
2) Take your main texture in your texturing app, make a copy of it but apply damage to it. (This usually just involves lots of black areas, but for really good debris you'll want to mix in areas of a damage image. Pics of scaffolding work quite well here.)
3) Pick a relatively unseen area of the texture and add a texture like "DocTile6" (which is the usual debris texture) to it. This bit will be the part that you texture all the internal parts of the chunks with.
4) Back in the 3d app, move everything else out of the centre of the scene and place your duplicate LOD1 above it.
5) Apply your debris texture to the duplicate.
6) Carefully select groups of faces that might actually crack apart when the ship explodes and separate them from the main object, moving it straight down to the centre of the scene where LOD0 used to be.
7) Fill in the 'hole' that is now on the inside of the chunk and map this area straight onto the DocTile6 bit of your texture.
8) Repeat the hole plugging and texturing process with the remainder of the hull.
9) Repeat steps 6-8 until you have turned the entire mass of the ship into debris.
Have a look at any of the HTL ships I've worked on for an example of what this results in in-game. The ships crack apart and hurl chunks of themselves in all directions, which looks VASTLY better than the cases where 90% of the ships mass simply vapourises leaving only a few odds and ends.
