Hi Chung----firstly, I'm not at all jealous of where you live !!
With regard to your question,-- hopefully folks who know a lot more than me about plants, fertilizer use, and CO2 use, will be along soon to guide you there.
Regarding the Neo shrimp, ( which I am experienced with), the first thing I would say is that a well established tank, ( let's say 2 months plus), is hugely beneficial to their care and ability of their offspring to survive on the bio-film that takes time to develop (aided by Bacter AE, or similar product ).----not saying it's vital, just desirable.
Maybe a good plan would be to let your new plants settle down for a suitable period and start active growth, and then remove about 10 of your colony from the existing tank and transfer them.
If they're still around after a couple of weeks, you should be fine.-----but if they're not
, at least all has not been lost, and you can think again.
I don't know how big your colony is, but certainly I would transfer them in small batches, rather than dumping them all in at once.-- This will give time for your bacteria to increase and adapt to the extra bio--load created by any added shrimp food you introduce etc.
Anyway, good luck, and I reckon a 20 gallon planted aquarium with a thriving yellow Neo colony would be very pleasing to the eye.