Hello again. Somehow i managed to make it work on mixamo. I used blender to exclude the skeleton of the character and then exported it in OBJ instead of FBX and then it worked on mixamo. Hope it helps you. I'll try to make the texture without the long skirt too and then update here. Btw, thanks for replying, it helped a lot.
(Maybe you don't need to export to OBJ, because it can cause some problems with the texture on the engine you'll use it. maybe just excluding the eskeleton is ok)