They can be nippy,but I believe there are quite a few people here who keep them together in the tanks. A good planted tank would help prevent the nipping somewhat I'd assume. I don't keep danios anymore,but I'd say as long as they don't have a lot of wide open space to play chase they should be allright.
my danios (6) live in a community tank with several livebearers - never seen any fin nipping or agression. Perhaps it depends on the individual fish...
I had one old tank with Zebras Danios and guppies and they got along fine, Now on another tank which i am setting up, i got 4 Zebra Danios and 5 Guppies, they chase the guppies from time to time and go back on chasing one another, so far no casualties and Fins are healthy, but i am thinking about taking the danios out.