I think we agree that what you hear rules, so I recommend to try in practice.So you're saying that the Schumann resonators could remain? Sorry I'm confused.
But in theory, as long as you don’t have various Schumann or other resonators of different manufacturers in place which could be too much on a common frequency, I’d say yes, they can remain (theoretically). They won’t interfere with the Schnerzinger equipment in terms of overcompensation as newer Schnerzinger equipment only acts on existing noise.
Regarding Schumann resonators together with Schnerzinger Giga Cancelling, I assume Schnerzinger would also recommend to try by ear, what you like best and to repeat that with progressing use of Giga Cancelling devices. At the end you might prefer Schnerzinger only but inbetween there might be other phases. I just think it’s counter productive to use other devices with various fix HF noise suppression together with Schnerzinger, as you then have the disadvantages of the other devices paired with lesser efficiency of the Schnerzinger devices in this case due to overlapping.