Correct me if I'm wrong, but.....
Could I install a free float tube on a 9mm barrel/upper with a milled front sight base?
If I did so, would it be easy to remove the ff tube and separate the barrel from the upper for cleaning.  I am assuming that a 9mm is recoil operated instead of gas operated.  Hell if that is true, I don't even gas block/fsb.

I'm thinking of building a flat top 16" 9mm carbine/w fft

That is correct.......you can install a free-floating handguard in place of the two piece handguard.  Remove everything from the extension tube foward and install as suggested.