FLARM and similar systems use GPS altitude. The barometric pressure or altitude are irrelevant to the collision prediction. Thus adding a baro sensor is not needed, unless you want a more accurate "vario" function based on the data output from SoftRF into relevant software on another device.
ADS-B transmissions report barometric altitude. That is a bit of a problem for collision predictions regarding ADS-B traffic in SoftRF (if you add the ADS-B receiver module to SoftRF). My code handles it like this:
* If there is also a baro sensor attached to the SoftRF device, it uses that to convert baro altitude to GPS altitude.
* Otherwise it listens to some other types of ADS-B messages from aircraft that are not very different in altitude, some of those messages also report the difference between GPS and baro altitudes, that can be used to
convert baro altitude to GPS altitude.