IME any fish you put in this tank will eventually be killed with few exceptions. Another female may work, but this situation usually leads to one of them being beaten up and possibly dieing . One thing that could work is a smaller growing pleco like a bristlenose or a rubberlip provided with a cavre that the cons cant follow into. Thing is you'll rarely see the pleco, and they will eat the fry once the lights go out. Breeding cons are notoriously violent to anything in their vacinity. Giant Danios may work as the are pretty fast and can generally get out of harms way but you'll still loose a few here and there. Cories are best kept in groups and would likely be murdered by the cons during spawning and while guarding fry. Since the pretty much all the time with cons you'll probably want to avoid tank mates till you can get another larger tank, or another tank for other fish.