The spelling of /f/ is <f>. The /ʒ/ sound is spelled <ge>.
The contrast is between two fricatives, one voiceless and one voiced but well separated in the mouth. It is unlikely to cause a problem.
The /f/ sound can occur in any position in the syllable, but /ʒ/ only occurs finally or intervocalically, other than in foreign names or loan words such as gendarme. /ʒ/ is in any case a very rare phoneme, found in only 334 words in the source dictionary.
The figure for the mean density is too small to be worth calculating. The semantic loading is 50%.
roof rouge roofing rouging
John Higgins, Shaftesbury, January 2010.