It's a character setting, not something you can apply to the whole race. Your starting characters are likely identical in the FCS, so changes to the thralls would affect any non-thralls that you started with. You can check if this is true by looking at which character templates are included in the squad for the game start you used.

If you haven't made much progress in your save, you could duplicate the game start in the FCS and replace some of the characters with copies that can't speak. Set "named" to false for the new characters so you can identify which ones can't speak when customising them.

How do i apply it to a character if they're non-generic i.e. only two exist, I made them as start characters.

All you need to do is find the new thrall characters, and change the "dumb" value to True.

How do I apply this mod's affects to a mod thrall race via fcs?

SKREEEEEEEEEEEEEEEEE

@balbor.valis Yeah it should work with that mod or any of the generic prisoner recruiting mods.

will this work with the reprogram skeleton mod?

Very nice!