Would recommend updating any of the documents online, using your browser, rather than downloading and altering "offline" (and unless you really do need all members to be able to update documents, having them view only for the majority would cut down on chances of the one and only copy of a document being wiped out).
Software shouldn't be a problem as most of the major 'office' packages save to the same types of file format. So, for example, Microsoft Office can read and write OpenOffice documents and vice versa.
It's best to think of Drive as a medium, not as a filetype.
But, in the event of your Macs not being able to read/write to the existing files, you can always get Microsoft's Office package for Mac (though, that being said my son is sure OpenOffice should be able to do that anyway).
Can you not modify the documents online in Drive itself?
That is the best way as you can keep track of who has modified what when, and prevents the problem of two people downloading and modifying the same document that the same time, and then uploading, resulting in two slightly different copies of the document.
For you do it online also means pc/mac doesn't matter, and you can use features like commenting properly.
I can't answer your questions on drive, but you can open Microsoft documents in openoffice. However, if you use fancy formatting, wordart etc then quite often the conversion doesn't work properly so you wouldn't see the wordart in openoffice for example. Also, macros aren't available in openoffice if you use those.
I am involved in running a voluntary orgsnisation with about 70 members. Currently our paperwork is a mixture of old hard copies in folders, various electronic docs held by the president (me ATM) and our last president set up a Gmail account. Some of our shared docs are on Drive now and I would like to move all our stuff onto Drive.
What happens with docs in Drive if different people access and modify them using different software? Most people have PC (Word etc ) but my Oh is keen for us to start using a Mac (open Office). Some of the docs were created using Drive but most of them are MSoft Office which we download, modify then upload again. Will there be problems switching between MSoft and Open Office?