Long Fin Black Skirted Tetra problem

Question
Got a 12 gallon tank (Eclipse 12) and we need advice. My son picked a long
fin black skirted tetra & 2 gold skirt tetras to start. The gold skirts are picking
on the long fin black skirt--it spends most of its time hiding. Will adding 1 or
2 long fin black skirts help, or should we pick different fish to complete the
tank? Ideally my son wants to add 2 longfin black skirts, a platy, and a cory
catfish-thoughts? We've read conflicting reports about whether or not platys
and corys need to have more than one of their own kind.

Thanks for all suggestions!

Answer
TQ,
It will not hurt to add more tetra;s.
I would try to keep them in odd #'s. 3-5-9 etc.
For some reason they do better.
Platies and Corys are fine alone, however they show better and seem to be more active when there are two or more.
Any of the "colored" tetras, the blackskirts not withstanding as that is their true coloration ,will go together. The pink, blue , green, gold etc are all the same fish, just dyed to a certain color.
Be advised that over time the color will fade and they will revert to their natural state which is white.
If you have additional questions, just ask.