Looking at AR9 lower with shockwave "brace". Can I put any size barreled upper on this ? Looking at 10.5 w/flash hider.

Will this be too long or is a pistol a pistol with brace and any barrel will work.

Federally, any upper. Some states might have pistol barrel length limits, though.

26 inches plus makes it a "firearm" and not a pistol.

The short version.  Someone will come along and either say I am wrong or give specific details I was too lazy to type!

You were right that you were wrong.

Adding a verticle grip to a pistol over 26 inches does not make it a "firearm."

Being over 26 inches and being a "firearm" one can install a verticle grip without the need of a $5.00 tax stamp for an "any other weapon".

To add a verticle grip to a pistol requires a $5.00 tax stamp for an “any other weapon”

This explains it all.

That's exactly what it does.

Read the letter in your link a little more carefully, mainly the last block of text on page 2.  Also read the federal definition of a "pistol", notice it has no stated maximum OAL.  A pistol with a 16" barrel is still a pistol.  However, pistols are "designed to be held by a single hand", so adding a VFG changes that and takes it out of the realm of a "pistol".  If you add the VFG and the pistol in under 26" OAL, the ATF contends it is then "concealable", and would be an AOW.  If the pistol is 26" or greater in OAL and you add a VFG, it now fits none of the other federal sub-definitions of a firearm, so it is simply regulated as a "firearm".