Depends on what you mean by optimizing. Textures can be "optimized" without issues, the .dds image format for textures is standardized and textures cannot cause CTDs or similar problems by themselves. Normally, when it comes to textures, "optimizing" means compressing them in the most efficient way. For textures without transparency (or only 1bit alpha channel, so every pixel is either completely transparent or completely opaque, without transitions), DXT1 compression is best, for textures with transparency, it's DXT5. Unlike quite a lot of people seem to believe, the only difference between DXT1 and DXT5 is the alpha channel, the quality of the compression is exactly the same, so there is no reason to save textures without transparency in DXT5, because that would make the file twice as big with absolutely no visual difference. So one think Optimizer Textures does is scan the alpha channels of the textures and makes sure the textures are properly compressed. It can also add mipmaps to a texture if it doesn't have them and convert textures which are saved in a different, less efficient format than .dds (like .tga).
Sometimes, compression is not wanted, though, as it does slightly reduce the image quality. Especially normal maps can in some cases look noticeably worse with compression than without. Most textures you get from mods are already compressed, though, and the compression cannot be reversed.
That being said, the 8K texture from Kvatch HD is already compressed ideally, so there is no way to reduce the file size but keep the resolution. Optimizer Textures has a resize option, though. As can be seen in the first screenshot on the download page, you can specify that textures above a certain size will be downsized to half their width and height, resulting in a decrease in file size (and thus, VRAM consumption) to 1/4 of the original file (because the amount of pixels in the image is reduced to 1/4 by cutting width and height in half, if that's not clear). I remember noticing this texture myself, and I came to the conclusion that 2k is more than enough for this texture, so feel free to downsize it twice, I think you'd hardly notice any difference, if at all.
Strangely enough, this texture is only 4k in the main file, but 8k in the update, so I am not sure that it really is an oversight. Even more puzzling, though, the normal map that belongs to it is only 256x256 pixels big - I am not sure if this is even vanilla size! So basically you will have a super high resolution wall with probably finer detail than your screen can even display, with a normal map that can in no way match the detail of the diffuse map (the main texture). Anyway, the mod still looks quite good. There are a few more 4k textures you could downsize if you like, but the rest are pretty alright I think.
As for meshes, yes, that is a different thing entirely. In some cases, this can cause errors (not sure about the "already optimized meshes" thing myself, though), possibly even CTDs. Better not try optimizing all at once, but go folder by folder and check in between if everything is alright. Of course, you cannot check all meshes at once in the game, so problems may still appear later. I believe I only optimized the vanilla meshes with PyFFi, and not all of them. I don't think it is a good idea to just optimize every mod you download and install, it may cause more problems than it solves.