Everywhere I've read anything about firefish it says not to keep them together. On rare occasion I've seen some people keeping just a pair together or more in the same tank, not meeting the minimum requirements for keeping more than a single. Does this rule of not having more than one in a tank apply to the different sub-types to? Like having 1 Firefish + 1 Purple Firefish? or even for a daring person, a Helfrichi + Another Firefish?