If you use BX CSS Expansion from the Brawlex thread's original post, you can add BX characters to random in CSSRoster.dat. This file replaces the functionality of custom CSS and custom random codes.

The Brawlex config utility can help with editing CSSRoster.dat. Simply increase the number of available icons to the amount you want (max 100) and assign the ID number to CSSSlot##.dat's number. The ID to call random should be placed here too. Then repeat the same thing for the random list (minus the random ID, as random shouldn't be calling random).

I understand the first part (as it is what I do) but I don't understand the random list part ^^;

Never mind I figured it out, thanks!

I wanted to know if it was possible to make BrawlEX characters be picked by random as I like to set up matches where I just fight CPU characters chosen randomly but none of my BrawlEx clones are picked. Any help is appreciated

How to solve: In BrawlEx Config Utility, modify the CSSRoster.dat file, click on random and add there the IDs of your characters.