How much clearance should I have between a lightweight barrel with a shaved FSB and a free float rail to account for barrel whip/harmonics?
A shaved FSB should be every bit as low as a lo-pro gas block. Since handguards are made to clear those gas blocks you shouldn’t have any issue. If you shave the FSB so there is about 3/32" of material around the tube you should be good to go on any handguard out there.
ETA: I just looked at the last one I did, just by cutting off the sight portion you end up with the same height as a lo-pro.
My Fortis rail looks like it should clear the shaved FSB, but my new Centurion CMR uses a proprietary barrel nut that Stickman says will no way fit over the gas block. And since it was for a pinned 14.5" upper, I have to start over with a stripped barrel or cut the old nut and FSB off. Luckily I have another rail that will work so the CMR will have to be third in the project queue.
The Fortis is a good rail and it will clear the gas block with no issue. There is a ton of room above the gas block on it. I would use it on your 14.5. I would get a 16" and run it with the CMR. There is no barrel nut that will go over a gas block. 
Thanks for the reassuring Fortis pic.
For the other, a 16" would be easier, but I have enough of them and Stick’s shot of a 14" CMR on the shorter barrel gave me an urgent jones.