"I know the first thing I'll need to do is install the Cambria font on this Mac so I'll be compatible with all the folks who receive my spreadsheets and use Excel."
While having Cambria installed on your Mac will prevent seeing the warning every time you import an Excel document, it won't in itself make your spreadsheets (fully) compatible with 'all the folks who...use Excel.'
Although you may think of it as "opening" an Excel document, the more accurate description is that you are 'importing and translating' an Excel document. Once the document is open in Numbers, it is no longer an Excel document, and it is no longer exactly the same as it was. When you export that Numbers document to Excel format, it goes through a reverse translation. Neither translation is perfect—it cannot be, as there are features in excel that are not supported i Numbers, features in Numbers that are not supported in Excel, and features in both applications that are implemented differently in each.
If your spreadsheets remain fairly simple, back and forth compatibility could be 'pretty good.'
If you want FULL compatibility with Excel (on Windows), you'll get much closer to it using Excel for Mac.
Regards,
Barry