lods don't need to have the same subobjects, you don't need even the turretts, if you don't want them.
FS2 supports 4 LODs (actually you can convert 6 LODS, but I never really understood if FS2 can use more than 4...). Lod0 is the high poly one, Lod3 is the less detailed. You see LOD3 only when it's very far, so don't care too much about it's shape: 100 polys will be enough.
You can use completely different maps for the different lods, and you also should. LOD3, for example, can be mapped with a single simple small texture (256x256) repeated all over the mesh, and for the other lods you can just use the same textures of lod0, but resized, expecially if you built them welding vertices of lod0, and saving the uvcoords.
And in TS it is very simple to edit hierachy, using the scene editor (keyframe editor in some versions), the icon is placed on the upper left menu bar.
check this tutorial (note it is a fs2 tutorial, not an fso one), and bob's pcs guide, for hierachy examples:
http://underworld.fortunecity.com/pacman/106/fs2mods/shipcreationguide/index.html bob's guide:
http://freespace.volitionwatch.com/blackwater/fstut/fstut_index01.htm
