Mike,

I did about the same, swapped my ever getting older FC28's for (used) FX30's. Indeed because of age and the 8 channel PPM limit. Basically to sit out the waiting period to the tray transmitter with more suitable TX's.

I set my transmitter modulation in my TXes in the FASSTmode, and my receivers with GC into (Manual) WEATRONIC modulation.
Works like a breeze, The 7.5 voltage in the FX30 does not seem to affect the performance of the WEA unit, I am a tug pilot and we need RANGE (Altitude).

However I did not (yet)get the audio output of a combo GPS unit/MUX box/IAS tube telling me what I like. But that might be some crooked software setting problem I have to delve into with the help of WEA.