A well i always keep new fish on lockdown in a 100L aquarium till i am sure is all ok(2 weeks usually).
As for taking fish from others... I really would not since my 92 gallon was actually made to house 5 Discus
BUT i just cant let someone dump the fish in the toilet so my dream Discus
aquarium has turned into a... home for the abandoned/homeless fish shelter :D.
My worry was the liter/fish thingy as i read that for a angelsifh u need 50 liters and a Neon Tetra
needs about 6 liters each thus 100 liters gone for the angelfish and another 180 gone for the 30 tetras it's 280 liters already leaving 70 liters for the 5 gourami+5 tetrazona+4 boesemani and the algae eaters.
Not to mention the 120 liters the 20 new tetras would need.
But i guess they will be ok even with less then the perfect liter/fish ballance as if you see the aquarium u'll notice that there are parts of it totally empty since all the fish seem to enjoy only the left side of the tank for some unknown reason...
Here's a picture of it, not even half finished but i am waiting for the rest of the plants and for a serious CO2 sistem as the one u can see in the pic is not even 10% enough for the aquarium but as things move slow here all one can do is wait and hope :)
Enjoy, awaiting remarks be they positive or negative.