I've not had luck keeping skunk cleaners and peppermint shrimp together. For whatever reason, the skunks survive and the peppermints die after about a month or so. This is in our 90g, where we had 2 skunk cleaners and 4 peppermints (on the recommendation that peppermints do better in groups). We had to replace our 2 skunk cleaners once in almost 4 years, but have gone through 3 different groups of peppermints. I've never seem them fight for food, and have seen both actively eat at feeding time, so I'm not sure what's going on or when it's going on, although it's probably happening at night.