I just downloaded the latest FSDT GSX installer (1.9.0.6) and got a pleasant surprise. First it detected P3Dv2 (nothing new there), and asked me if I wated to install it there, I had not done so yet, so I installed it there.
Then it detected FSX-SE, and asked if I wanted installed there, so I said yes.
On the third run of the installer, it installed into FSX.

Seing as it found all three installations and ask if I wanted it installed in them all, it must be compatible with FSX-SE already. ;D