It will be compatible with Black shark - Black shark will receive a compatibility patch for Warthog (i.e. BS gets all of warthogs improvements).
FC2 will not be compatible according to current plans (although you will most likely be given the option to retain your current version of BS to fly with FC2, and the update will be installed in another folder).
Q: Will DCS: A-10C Warthog be online-compatible with DCS: Black Shark and Flaming Cliffs 2?
A: We hope to be able to release a compatibilty patch for DCS: Black Shark after release. Compatibility with FC2 is unlikely.
Nate