>IOWs I want to use VFP7 beta2 to work with, but will it make anything incompatible with VFP6?

Don't open menus in VFP7 (it will want to convert them) and don't turn DBC events on. Other than that (obviously) don't use any of the new syntax, and make sure you compile your code in VFP6 before you run it. If you've got an inputbox.prg that worked in VFP6, and you use VFP7 to write the code and run it in VFP6, it will tell you the feature is not available.