Loaches, just about all varieties, are schooling fish and great for community tanks, provided they have buddies or they get depressed; nothing's ever stated anything about aggression even with a loner. For loaches the minimum seems to be at least 4 to a school. You can look at
www.loaches.com for more species specific info. I really want clown loaches, so they're the only species I've done any extensive research on. Loaches are snail-eaters, though!
As far as danios being nippers, I want some of these, too, and I never read anything stating they would be agressive at all. It might be possible if you don't have enough of them, they apparently are a minimum of 6 to a school. They also need lots of room to swim, as do most types of loaches, so a long tank is preferable, with lots of good water flow. I have a feeling whoever initiated the rumor that danios are nippers either bought an agressive breed without researching it or has some really depressed, lonely fish or the other fished being nipped on are just smaller and the danios are just thinking they are food. All I hear, though, is that they are good for community tanks and are very active. Loaches seem to be skidish and need their hiding places, but are voracious eaters and even when they're full, they beg for more food.