I would move the whole spawn to the bigger tank all at the same time and remove the over aggressive ones as you see them.
Females usually pick the dominate male and they line up to spawn with him-the less dominate or more passive males sit back and stay out of his way so not to make him mad and he kicks their back-side....lol......sometimes the less dominate males will not fin out very fast/good and will maintain shorter fins...not all but some do.
As long as the brothers are never separated it may work in a 20gal-but a 55g would work better and increase your odds of success with keeping two male together...you won't know unless you try-just be ready with a back-up plan
Hey now that i have a succsessful male/ female tank im wondering how you keep two males together? Im planning like a 20 heavily planted and two males and maybe 7 or so females. Also like planting it extremly heavily planted down the middle acting a wall kinda. These males will be from my current spawn . Any advice?
Thats right.....it is kinda a non-issue and the beauty of active growing live plants-often with live plants they will use the ammonia before it converts and you may never see nitrates and so that makes it hard to know if you are cycled or not. Plants take care of the water-you still need to make some water changes but once a NPT matures those can be decreased based on stocking-In my mature NPT some with filters and some without get 1-2 times a year water changes at most-I do top off with fresh water every week because I have open tops and have lots of evaporation....but no regular water changes, no health or fin problems...happy thriving fish and inverts.....this is for heavy planted tanks not tanks with plants...big difference.....
I wouldn't recommend adding Danios to a 4g even to cycle-with soil and live plants you don't have to worry so much about the nitrogen cycle-the active growth of the plants should take care of the water.