Version 8 uses xml (xmod) structure files (mod)rather than text files for various reasons. The main ones bieng that xml is becoming a generally accepted future standard and allows easier formatting of model file information displays and prints and the newly introduced Audit Trail.
So xmod is the only file type that V8 supports.
This should not be a problem for V7 since the xmod model files were introduced in V7 (other than the usual warning that new features and functions will not work).
It does mean that supporting V6 and older versions with V8 is not an option. However, if you consider how much more there is in V8 compared to V6 this does not seem to be an unreasonable constraint. Also if the rest of your usage is already at V7 level you do not have a problem to use suitable xmod files. (Also xprj files - the same reasons.)
I hope this helps you.