For advantageous and courageous, you are just adding "ous" because the words "courage" and "advantage" already end in e. If you look at the noun forms of the others... Contagion, hilarity, and ambiguity... They have the same rule, after dropping the suffixes. However the adjective forms are more common than the noun forms. I think for the most part this is just something you have to learn with the word.