It's always a good idea to wait at least a month between fish additions, especially if this is a new tank. The biofilter needs time for bacteria to catch up and stabalize after additions of new waste producing animals.
If you are properly quarantining animals prior to introduction, then you could easily get them now and after the 4 week QT
time is over. The display tank should be capable of handling the new fish by that time.
Make sure your display tank and QT
are properly covered with eggcrate material or the like. Firefish are prone to carpet surfing.