Merged projects and default.css

Using Robohelp 8.0.2 for WebHelp

I am new to merging projects. Which elements of the project must be present in both parent and child projects, and how do I introduce child project-specific elements.

For example.  If my teammate modifies the "default.css" in a child project, will the parent project "default.css" override theirs? i.e. Will added styles in the child project be defaulted to normal?  Shoud each child project have its own childdefault.css applied to all topics?

If my teammate modifies one of the Master Pages to include common button images that the rest of us don't need, I don't expect it will impact the rest of us, since it's only applied at time of topic creation.  Is that right?

We've decided to maintain a common glossary.  Is the glossary part of every child and parent project?  Is it possible to have separate glossaries, or does the parent project glossary override any child project glossary?

I understand that all parent and child projects must use the same skin. Is that correct?

Thanks for any insight!  No doubt I can test each of these things, but I'm sure you know other things I haven't even asked about.