Don't flush the danio! That is very irresponsable behavior, to be a good fish keeper you have to take proper responsability over your fish and part of that means not flushing them down the toilet when you mix the wrong size groups together in too smaller tank and get issues.
Danios are very hyperactive fish that can get very nippy when in too smaller tank or group- you have both im sorry to say. Danios need to be in groups of at least 5 and need a tank of at least 20gallons, so this explains why your danios aern't behaving well in the 10gal.
I personally suggest you either upgrade your tank and get a 25gallon tank and 2-3more danios or go to your lfs(local fish store) and give them the danios for free, at least that way they won't have to die.
Mollys can have up to 100fry, and these can take up to a year to fully mature and thus you are going to need at least a 10gallon tank if you have 15-20 or more fry, so you must decide now on wether you can handle the fry.