1. There isn't any limit I know of, but as pointed out by problems with Mrise, using large textures can overload and crash the game. He had many 512x and 1024x textures, and had to convert to 256x in order to stop the game from crashing. 512x can be used for buildings, but otherwise, larger than that is unnecessary at Glest's zoom level.
2. At the moment, MegaGlest is the only one with true cliffs, though they can be replicated roughly by having a sharp drop-off and using a ground texture, though MegaGlest does this automatically and with a sharper drop-off when the height difference is large enough.
3. Depends on what you mean. There is 10 different map objects, 5 different resources (actually a part of the techtree though), and 5 different surface textures (6 with the optional cliff texture). However, you can have an unlimited (?) number of these objects, such as 10 different variations of trees (object 1).
4. GAE has specular and normal mapping. This is done by putting _normal or _spec (?) to the end of a filename with the same name as the texture. For example, if a unit's texture is called "my_texture.png", then the normal map would be "my_texture_normal.png". Note that you'd need the GAE 0.4 beta to use normal or specular mapping. The G3D hack tool would also be helpful for that.
5. Besides the fog in tilesets which has always been possible, there isn't a true way to have the entire map have a particle effect, though MegaGlest supports particles on tileset objects, which can replicate it fairly well.
6. This is done by having a move skill, but no move command. If there is no move skill, the hold position command will not rotate the unit.
7. There's been discussion in the GAE board about it, but it's a big change, and a long way in the future.
Since you've been gone a while, you may be interested to know that MG and GAE will be merging (if you didn't already know).