Can you earth ground all different voltages of a system together?
Electrically speaking: Yes

In almost all cases, you can connect protective earths together.  There are some rare cases where you're working with small analogue signals and the protective earths double as shields where you might not want this, as it would introduce extra noise.

But...

It sounds to me like you are talking about building wiring.  If that is the case, then it's not enough to get it right electrically, it also has to meet the requirements of your local building codes.  Building codes make sure everything is done properly, but they also make sure everything is the same, so other electricians don't get nasty surprises. Sometimes they have rules which don't really make sense from an electrical engineering standpoint, but are enforced anyway.  Building codes also vary a lot from jurisdiction to jurisdiction, so you need to check the ones for your area.  Either speak to the appropriate authorities directly, or try posting over on the DIY stack exchange (make sure you say what jurisdiction you are in).