Scenario-1:

Yes. You can open both versions of excel  files in each version of excel. But
here you've to do one thing. Do not open both files by directly double clicking
on them. I doubt both will open in excel 2007. Rather you open both the excels
from your start menu and then from file menu open the respective versions of the
files in respective excels.
One thing I want to tell you here is that, excel 2007 is having an option to
save the file in different formats, even you can save in 97-2003 excel format.
But if you save as excel 2007 format then the file extension will be .xlsx. At
the last you will have 'x' added with their previous version etensions. this is
applicable for word and powerpoint too.

Scenaroi-2:

Yes. You can do this. Even more you can do. instead of opening excel 2003 from
start menu, you open excel 2007 and from file menu you can open both WB2003.xls
and WB2007.xlsx in excel 2007 inteslf. Otherwise you can double click the file,
and it will open in excel 2007 whether it is WB2003.xls or WB2007.xlsx.