Probably a basic question here.

I have a 24 volt solar system that I convert to 12 volts for a fuse box. I am using a DC 24V to DC 12V 40A 480W Step-Down Transformer, if that even matters. Generally, the fuse box has a negative bus but in this case, it does not. All of the components are connected to a common, negative, bus bar at 24 volts.

So, the questions are:
1. can I connect a 12v and 24v negative on the common negative bus bar?
2. can I connect the negative in and out from the Step-Down Transformer to the same bus bar?
3. For 12 volt appliances, I have generally connected the negative to the fuse box. Would I now connect them to the common bus bar?

 

It would be a good idea to show a circuit.
But as I understand it, there will be no problem combining the -Ve connections.
I take it your 24V to 12V "transformer" is a switch mode regulator. A transformer is an AC operated device, 2 or more coils wound onto a common core.