I am planning to upgrade existing SQ8.9 LTS to latest SQ9.9 LTS.
Since for some reasons I need to keep referencing legacy SonarCFamily plugin (6.7.0), I want to know if SQ9.9 is working fine with this legacy 6.7.0 plugin?
It is not supported to try and use a previous analyzer version on a newer version of SonarQube. You should not try to do so.
Why are you trying to do so? AKA, what are the “some reasons”