To answer one of the questions...
All of the textures for official characters are encoded into their respective .sexsim files and aren't available for regular editing.
Your best bet is to start with the character textures from
this page (look for the link that says "Here is a zip file containing base texture files"). You can load those base textures into your graphics editing program of choice and tweak them from there to get exactly the coloration you want.
For info on your other questions, make sure to fully read through all of the bold highlighted tutorials from
here.