Definitely too many at once, especially given that some of those can be larger fish. Even in an established tank, that bio load is going to reek havoc on your system.
So a few notes:
The firefish can be a bit shy, so it'll be best to not overwhelm that one. A 6 line can get aggressive, so that may be a bit of a risk - it's been hit and miss for me.
I'd also be concerned with the butterfly. They can be tough to feed, so i'd probably QT that one separate, and if this isn't an established tank, by all means WAIT. Your success rate is gonna be low otherwise.
The hippo is also a pretty big fish (potentially) as is the foxface (mines new and about twice the size of the dwarf angels that are maybe a year or two old). Again, keep bio load in mind and space these bigger ones out so you don't have a disasterous ammonia/nitrite spike and wipe out the tank.
If you space them out and isolate the larger fish, 20 should be perfect. I QTd all mine in a 5-10g tank (maybe 2-3 fish at a time) and it went well. If you're medicating (I treat mine with prazipro and methelyne blue) the new ones too, the smaller tank will likely make that easier. Also makes a more usable tank in the future for hospitals/new additions!