I've had breeding pairs of cons with a few different types of fish including a silver aro and red belly piranha. One thing the cons are real good about sneak attacks from below and behind. This behavior is very stressful to most types of fish.
In my 5x2x2 i had a pair of cons and they took over half the tank... they were only like 3 inches and they were fighting off everything even my 13inch sailfin even though he didn't give a fudge about them but i used to really like those fish but now i would rather flush em.