For as long as I remember there is/was a philosophy that 'if it fits, it works'! GE/Murry,Bryant,etc, etc. Some even went to the extreeme of removing the rejection tab on twins to get them in. Heck, I saw a 42 circuit panel with 42 twins installed, and no cover due to the excessive # of conductors. (That was "only a temp, sir")
Bottom line, one added circuit in an existing panel, brand 'x' breaker, might get by. However, do a service upgrade, or a new install....all must be of same mfg.
