I agree I got some cories over 5 years ago. 3 houses later and 3 tanks later they are still alive.I'm more worried than I was.
cories should survive far longer than a month, as should any fish you find in the trade. my oldest cories right now are over 10 years old.
in the tank you have now, add the rest (4) of the cories and it's full. no more fish.
did you cycle the tank before you added any of these fish? if not you need to wait before adding the cories. it will be more harmful to have the bio waste from the extra fish than it will be to be lonely for the time it takes your tank to cycle.