Be careful with the word "always" here. As they say, you should never say always except sometimes.
Your rule is good if they are in the same period. But when dealing with atoms of different periods, the bond length depends on the atomic radii as much, if not more than the type of bond. For instance, a C=S double bond can range from 1.55 Angstroms to 1.61 Angstroms while a C-C single bond between two sp3-hybridized carbons typically falls around 1.42 Angstroms.