My guess going on protocol, not on antenna.
Have experience on one now 50 y old C172 avionics upgrade done 2 years ago:
System consist of GTN750+G5+G5+GMU11+GAD29B+Trig31+old AP+and the rest of old steam gauges without vacuum.
For example GTN was not able to communicate Alt on non Gar. dedicated protocol to Trig in way to complete ADS B out.
Position yes, Alt no...
Therefore an extra RS232 line going from G5 was necessary, but that limits other functions.
GAD29B interface to non Garmin AP was another trouble, still need to be resolved by replacement of whole AP
- paper work is another trouble/major modification
Conclusion from that, yes you can mix Gar with other, but regarding the compatibility level, be ready on some $$$ surprises...