For the QC problem, post that in Models/Skins section with the QC log, and the text inside the QC file (open with Notepad). People there can help you out.
It depends on what the problem is in your .QC file. There are several reasons. It depends on your model. To change skins, just get the Half-Life model viewer, go to Textures, press import, and then you can import your skin.
NOTE: Textures only work in HL if they have these settings:
256 x 256 size or 512 x 512 size.
Indexed color.
That's all.