What do you mean by "get place under 500 MB"?
under 500MB ram usage?
under 500MB hard drive usage?
under 500MB size of the iso?
What are your hardware specs? (CPU, RAM, storage, video)
And what is the specific performance problem with Mint Xfce and/or Fuduntu that you are trying to fix with a lightweight distro?
Computers can be slow in a lot of different ways (slow boot, slow internet, laggy applications, choppy multimedia performance, low FPS in some game) and the solution depends on the problem.
Mint Xfce and Fuduntu are two of my favorite distros, and I find both of them to be very
fast and lightweight.