In my experience ground loops on transceiver to CPU based units are only an issue in TX when using "longer" wires.
For reference I used a second Arduino Nano mounted inside the uBitx and all signal cables not on the board, that is that are wired to other parts of the unit, are shielded audio cables (microphone type, 2mm or so diameter cables).
I used the Nano to inject an audio signal into the audio (after low pass filtering) for my auto tuner and no audio harmonics or hum was audible on another receiver in the SSB passband.
I also used an analog input connected to the microphone input as a VOX detector and again I can't hear anything on another radio at normal volume.
In RX it should not be an issue in any case if your device is mounted inside or connected via short cables to the unit.
I would use a separate supply voltage regulator for two reasons: the load on the existing regulator would be increased significantly requiring a good heatsink and this gives you the opportunity to ensure a good filtering between the Raduino and the Teensy supply and prevent any feedback/bleed through that way. It is what I did in my rig.
Regarding the usage of the Teensy USB that is a good idea. Just a reminder that some (most?) Nanos keep the USB chip powered up when supplied from the internal supply (that is even if the Nano's USB is not connected) and therefore inject a TTL logical zero (+5VDC) when idle. Therefore you need to make sure you pull the digital line D1 (Rx) to a low enough voltage to ensure you get a solid TTL logical one (0VDC) when communicating with the Teensy.
See an example of that issue on this post (and the previous posts in the thread): https://groups.io/g/BITX20/message/66289
Hope that helps.
All the best,
73, John (VK2ETA)