I don’t believe FN makes a pistol version of either the SCAR 17 or SCAR 16. Therefore, putting a barrel less than 16” on it, regardless if it has a brace or not, would make it an SBR and would have to be registered with the ATF.
This would apply to AR platform rifles as well. Factory receivers that leave as rifles have to stay as rifles unless registered. Simply putting a brace on it doesn’t make it a pistol
Make sure you have the pistol brace installed BEFORE you install the barrel assembly.