I am not aware of any standard that will prevent you from connecting grounds of different power supplies ..
Of course, you have to implement safety precautions not to expose users to dangerous high voltages ..
As an example have a look at light dimmers, touch-button light switches (here you touch mains through some MΩ resistor) etc. etc.
They have mixed low and high voltage ground and to keep their size small, no isolation transformers or optocouplers are used ..
However, my advice is that whenever it is possible always use proper isolation ..
Regards,
IanP