Starling 2 Max is designed for 4S batteries only (two 2S Li-Ion battery packs, which are connected in series inside the vehicle). The Mini ESC, which also acts as a power source for VOXL2 inside the Starling 2 Max, is rated at least for 4S input (depending on the version of the ESC).
There are several versions of the Mini ESC to provide the users some flexibility, in particular the voltage of the VOXL2 regulator (5V for VOXL2 and 3.8V for VOXL2 Mini). Since Starling2 Max uses a VOXL2, the ESC part number will be either M0129-5 or M0129-65.
An updated (6S) version of the Mini ESC was released after the release of Starling 2 Max (new ESC part numbers: M0129-63, M0129-65). This ESC update extended the supported voltage for the ESC (not for Starling 2 Max) from 4S to 6S. The 4S version of the ESC has been phased out because the 6S version outperforms the original version even at 4S voltages (due to better Mosfets), so the 6S version is an improvement over the original 4S version of the ESC, while being fully backwards compatible.
Using the 6S-rated ESC in the 4S-rated Starling 2 Max is completely safe.
If you use the Mini ESC outside of the Starling 2 Max, you need to verify the part number / sticker on the ESC before plugging in 6S battery (M0129-63, M0129-65 only)
The ESC variants and part numbers are documented here : https://docs.modalai.com/voxl-mini-esc-datasheet/
Alex