I put two small black whiptails in my community tank which houses female betta, dwarf gourami, guppies, kuhli loaches, otos and choc zebra.
They were only 3.5cm and 5cm so very small and skinny!
The smaller one disappeared after a day or so and I'm pretty sure the other whiptail killed it and then all the fish then ate it.
The other fish have not bothered the 5cm one at all and they all get along just fine, which is why I think the 5cm killed the 3.5cm one. When I bought them, the seller bagged them up separately for me in case they turned on each other in the bag .. so that was my first warning! He said they shoudl be fine together in a community tank with lots of hidey places .. but I think I wouldn't buy two together again unless they were both the same size.
I have an almost full sized black whiptail in another tank. he was put in as a baby as well, with a male betta and two platy - no problems there either.
here are the two babies, the one on the glass was the tinier one.
here is my adult whiptail who interestingly has become a creamy brown colour.