To be a bit clearer on the side rail mounting:
If you use both Flarm antennas (A and B) on either side of the canopy then your Flarm reception should be well balanced.
However, your Flarm transmission would be skewed to one side as only the Flarm A antenna transmits. The Flarm B antenna is receive only.

Mounting the ADSB/PCAS antenna on either side rail should not be an issue as the transponders are transmitting at very high power levels (150-250 Watts)
So, you will be receiving more than enough signal from a transponder equipped aircraft a few miles away.