You don't say how many Danios or Widows you have, or what else is in the tank.
Normally, in a group of 6 or more, Danios and Widows will keep their attention on each other, rather then bother anyone else. Both species are capable of nipping, and the Widows in particular, (being front toothed), can take a chunk out of a fin with a small nip, (the Danios would need to work a bit harder to acheive the same result). Kept in small numbers, shoaling fish of any tytpe display atypical behaviour which at one extreme simply means they hide and at teh other, they become hyper aggressive monsters. Whatever, the stress on the fish usually shortens it's life drastically.
A different issue is adding new fish to an established tank. Frequently, when adding new fish, the existing occupants, who have already sorted out their pecking order and "place" in the tank, will chase the new fish about until the new pecking order is sorted out. That is quite normal and doesn't usually cause any damage.
Harlequins are very peaceful fish, and are quite an easy target for bullying. If they are not being damaged, and you schools of the other species are large enough, I'd put them back and leave it for a few days to see if it settles down. Obviously, the more hiding places you have the better. If after a few days they are skulking around out of sight or have obvious nips from their fins, then take them out again.
I have always found Danios to be chasers.
