1) Can you give me an indication of the polygon amount that is to be used?

I recommend using 750 polys or less for props/portable items/weapons. For characters, keep it under 2500. See this page for why.

2) Can I use other formats besides Targa for textures? Can I use any size?

Vampire only uses 24-bit uncompressed Targa format for textures, and the dimensions must be a power of 2 [up to 256x256 maximum] because of the way video cards handle textures. It helps to keep textures small because many cards have limited memory on board, and it keeps download sizes reasonable. I design textures in a size of 512x512 and scale it down to what I need when I'm done.

3) What's the deal with these 4 different weapon files?

These correspond to the 4 main skeletons used by various characters in the game for animations.

skeleton_0: 40 bones, 'normal male' i.e. Christof, Wilhem, etc.
skeleton_1: 35 bones, 'large male' i.e. HeavyG, Erik, Allesandro, etc.
skeleton_3: 48 bones, 'normal female', i.e. Serena, Zil, Kazi, etc.
skeleton_4: 37 bones, 'skinny male', i.e. Samuel, etc.

A weapon model, portable item, etc. needs a version attached to each skeleton, so it will show up when in use. Another version is attached to just one bone, and is placed on the ground for the players/ST to pick up.

The actual animation data itself is contained in .nad files, and currently can only be created with Alias|Wavefront's Maya 3D software, and the NODExporter tool that comes in the NOD Engine SDK. The .nms file determines what animations are played for each action that can be performed by a character. Animations should only be used with skeletons of the same type. (For example, HeavyG shares some of Erik's animations because both characters use skeleton_1 for movement.) Otherwise the animation may not play correctly due to the different number of bones.

4) How much detail should I add to models and skins?

That's a tough call. You want your models to look realistic with lots of blood spatters, bullet holes, rivets and screws.My view for Vampire models like props is that you shouldn't go too wild. Most props are small and hard to see in the game because of the darkness. Man-sized objects or larger are better. Save yourself some time modeling and add the detail to your skins to make them really stand out.

More to come...