i would add the firefish before you add the tang.
orange and purples get along fine in my experience.
as long as they are happy and not harassed, they are pretty hardy. i know the smaller they are the more fragile they seem, so maybe try for a larger one.
a friend had a tank with 6 or 7 orange firefish, and they schooled together, it was pretty neat to watch. mine is fine by himself (i think) but i am sure he would enjoy having a friend. i am hesitant to add a new one though, because there might be territory issues if i don't get lucky on randomly getting the opposite ---.
yes, i would love a helfrichi, but until i make a LOT more money, i can not afford to invest the money into livestock like that. firefish do jump, and seem to find overflows pretty well if they are not happy, and having a helfrichi jump out would be a shame, both in losing such an amazing ffish, and in the monetary loss.
i need someone to make a transgenic firefish that glows neon green, that would be nifty...