Since many people(mainly people with no experience with modelling) obviously don't know about this I decided to make a short guide.
First of all, no..there is no possible way how to make a mod that'll work for only one skin, which naturally means any skin in whole GTA SA can be replaced with any mod you like. You ask how?
Easily using any known program to replace files in GTA SA archives.
First step:Download the mod. Usually zipped or rar'ed.
Second step:Unzip, unrar the files on your desktop, for example.
Third step:Start the program(Spark, IMG tool, etc - it's all the same, it just looks different) and load the gta3.img archive.
Fourth step: (IMG TOOL ALTERNATIVE)
The files will be named wmyri.txd and wmyri.dff for example, but you know you use the skin, which files are named somyri.txd and somyri.dff, for example.
So what now?
In the program you use to install mods go to 'FIND'. Then search for somyri.txd, when the program finds it right click it, go for REPLACE and click on it. A new window will show up, it's for you now to search the mod you want, so when you find wmyri.txd(as the old example) double click on it and wait for the program to do it's job. After that do the same with .dff of the skin.
Fifth step:Click on rebuild archive, yet again wait for the program to do it's job, then go in game enjoy your new mod. Adios.
Pictures included in spoilers.COMPLETE LIST OF ALL OF THE AVAILABLE SKINS IN SAMP