If the danios didn't have ick and the sword still does, chances are you will introduce it to that tank also. Only good thing is danios are pretty resilient to ick. Ick will thrive on anything it touches, i.e. a net, plants, etc. Go to the pet store and get Some Jungle labs Ick Clear. Use as directed. May have to use more than once. Be resilient. Do the water changes as direted. The danios shouldn't be a problem to the swords unless they are really weak, but even then shouldn't be a problem. Melafix is not for ick. More for gill infections, missing scales, etc. I've had a ton of baby swords in the past and what you are doing for that fish sounds good. Be careful though after awhile they're like gremlins, they keep coming and coming and ........................................
Good luck!!
Good luck!!