My six line and lawnmower blenny are fine together in my 72g. My 6line is not aggressive though, he actually was bullied by a dottyback in my QT, so dottyback went to LFS. It does swim with my clowns (they are very peaceful) and inbetween them and among them and their RBTA, so it is not timid. Since I've had him my blue assessor comes out more. The six line has taken food from the blenny, but the blenny is a hog.