Will Family Tree Maker run on Windows Windows 7 or 8? You should find all you need to know here with regard to using FTM with Windows. The publishers are not flagging up anything that affects initial installation.
Are there any programs that will work on both PC and MAC? No, but it is easy to transfer information between a program on a MAC to a program on a PC (or vice versa) using the GEDCOM transfer method.
How can I keep different families in separate files? I would look seriously at Family Historian. Most programs allow you to start any number of files with a different tree in each. However in Family Historian it is also possible to keep track of different unrelated family groups using named lists but in the same file. For instance family A from area x could be identified separately from family B living in area Z. Having everyone in the same file avoids lots of duplication of work with other aspects of the program.
You may download a demo version of Family Historian at: http://www.my-history.co.uk/acatalog/Family_Historian.html#DEMV3
Is it possible for two people to work on the same file at the same time? No. However, two people who are working on the same family will tend to work on separate sections of tree and will swap and merge data on an occasional basis.
Can the Reunion 9 program transfer the data without installing XP into the Mac? Yes. You may transfer data backwards and forwards using the GEDCOM file transfer protocol between PCs and MACs