What are the repercussions of letting Skinny Malone live?

There aren't, really.

Leaving Skinny alive has no impact on the story. Darla gets to live but that also has no impact on the story. If you spare Skinny you can find him in the wasteland later in the game south-west of the South Boston Military Checkpoint, where has some additional dialogue.

I had Piper as my companion and she liked my choice to "end peacefully" during the dialog options. I'm assuming this is the only benefit to letting him live. Guess I don't get to loot their corpses though, but I'm sure anything good they have would just get replaced soon.