Can I use version X of TAO with version Y of ACE?
Only if X and Y were the versions of TAO and ACE that were released together. For example, ACE 5.1/TAO 1.1 or ACE 5.1.12/TAO 1.1.12. There is no attempt to make the interface between ACE and TAO backward (or forward) compatible. That being said, you may be able to coerce mismatched versions to work together, but it is not something you really want to do.