First, on the temperature. Going from cool to slightly warmer is much less of a problem than the reverse. But, too great a temp variation can shock as someone mentioned. Just how different are the two temperatures?
On the cycling/adding the danio, aside from the temp issue, you could add them at once, for 3 reasons. First, a 180 liter/50g tank has more volume so ammonia will dissipate into the water more. Second, you mention live plants. If these are reasonably fast growing, or include some floating plants, they will easily assimilate the ammonia produced by a few danio. With the live plants you should not be able to detect any ammonia or nitrite; test to be sure, but they should remain zero. And third, crashing around in the tank with a net is stressful to any fish, so grabbing them all at once is always better than several attempts when this is not going to cause issues.