New Guppy Bullying Nightmare

[JenLo]

Hi all, so I am back again with another emergency!
 
I have a 100L (22 gallon)  tank stocked with:
 
3 guppies
6 danios (mixed)
3 cardinal tetras
1 platy
1 plec
 
2 days ago, to that I added
 
2 guppies (2 white tail, 2 red)
2 platies
 
All was going well until I turned the tank light on this morning to find that both the red tailed guppies had lost their tails

. The poor things had shredded bits of clear tail left behind. Sadly one died and I had to euthanise the other.
 
The, 1 of the platies - a very bright one - started swimming around bashing into things and also passed away

.
 
Now I'm pretty sure they have all been victim to the older guppies in the tank fin nipping and bullying. Is this likely? But I am not too sure what to do about it. I just hope the remaining new fish can stand up for themselves. All of the guppies or boys - perhaps the source of the problem?
 
Any ideas??
 
Ammonia: 0ppm
Nitrites: 0ppm
Nitrates: 0ppm (planted tank)
PH 7.8
 
Thanks.