I've had 2 firefish in my 55 for a few months and just added a purple firefish over the weekend. All 3 get along great. They may chase eachother briefly but it seems playful and non-aggressive. Most of the time they just chill in front and "twitch". I'd say get some when your tanks ready.
I've had 2 of them for a couple of months and they share the same den and are always swimming together. I added mine at the same time, this way there wouldn't be any fights over territories.
+1
also may be just mine, but I've found that Purple Firefish are more aggressive than the common ones. Don't know anything about Helfrichi's - saw one once - never kept one.