Whenever I use the mod and try playing with a Alex Slim Type, it doesn't work, and acts as if I were just using a Steve Model with a Alex Model. I've tried fixing this on my own but couldn't find out what caused it.

Could you show me your skin file so that I can take a look at it?

Whenever I use the mod and try playing with a Alex Slim Type, it doesn't work, and acts as if I were just using a Steve Model with a Alex Model. I've tried fixing this on my own but couldn't find out what caused it.

This mod has been determining skin types by checking specific pixel is transparent or not.
It checks the pixel at (55, 20), which is occupied in Steve Model but not Alex Model.
Your skin file has that pixel occupied, which is fine in normal situation, but this mod requires that pixel to be transparent for it to treat your skin file as Alex Model.

I should also mention that the skin overlay works fine, but it may only be on Steve Skins, not sure.

Use an image editor like GIMP to edit the file, and make that pixel transparent.
To make sure, you can compare with other Alex type skins, make all the unused area transparent.

Mind explaining on how to make that one pixel transparent?

Am trying to turn the file into a .png so that I can use it, but am not sure how.

Nevermind, I figured it out

Am trying to turn the file into a .png so that I can use it, but am not sure how.