Hi Mike,
In those situations it is recommended to use an AHDSR modulator.
https://www.fmod.com/resources/documentation-studio?page=authoring-events.html#modulation
If you are looking to fade in/out individual instruments, then apply the AHDSR to the volume of those instruments (fade in/out will occur with triggering those instruments). If you are looking to fade in/out a track or event, then you can apply it to the volume of the (master) track (fade in/out will occur when the event starts/stops).
Thanks,
Richard