When you say “terrian” are you talking about a genSurf type terrain entity? Or just some random brushes you knocked together that you calling a terrain? If it’s the former then yes, you don’t need to flag the terrain as detail is the terrain entitys by default condition, so any rendering issues will likely be related to your shader I should think - it sounds like you’ve got a missing texture in there actually, the gray texture is wolf default for missing images/textures.
If you talking about a collection of brushes you’ve knocked together then you need to make sure they’re func_grouped and flagged detail as a whole unit. Again though you need to check what shaders you have applied to it for missing textures - as above gray = missing images/shaders/textures.
Single Player wolf has no issues with MD3 & ASE models unless they’re excessively large.