1). Take a character PAC file, use
BrawlBox to find, and export their texture data as PNG files, then use an image editing software to edit these textures. Save that, and replace it into the PAC, and save that as a PCS (as PACs in some cases).
2). As a PSA person myself, I'll have you know that there is alot of things to learn in this field, and I can't cover everything in 1 Post. Editing PSAs mostly uses a program called
Project Smash Attacks (This is where the term PSA is coined from).
3). Vertex hacks are edits to a character model by changing the vertex points. Making Ike's Hair look like Cloud's hair for example. Back in the day, this could only be accomplished by modelling programs like 3DS Max, but BrawlBox recently came with the option to edit vertex points. Though not as robust. The 2nd half of this question goes along with the answer above.