if the drain time is <3:30 your set's lowest diff must be normal or lowerif the mapper picks a song that is 3 minutes long, he could easily eliminate the need of a normal difficulty by extending the song by 30 seconds, if he is not skilled enough to make a decent extention using only the original song he could just add harumachi clover to the mp3, there would be nothing preventing him from doing this... same applies for the other lengths to eliminate the need of the hard or insane...
if the drain time is 3:30-4:30 your set's lowest diff must be hard or lower
if the drain time is 4:30-5min your set's lowest diff must be insane or lower
unless more specific rules are added for compilations like i said, the hard cut-off will be the only reasonable way for dividing ranked and approval.
any song can be shortened if the mapper wants to do everything alone without guest difficulties.