Let me share my experiences with my Polleni... had him now for just over 16 years roughly and in that time i've tried keeping him with tank mates on 3 occasions...
1st time: Polleni was bought with another chiclid at the same time (can't remember which species) they got along okay for a while since they were both small at the time till the other one past away about 6 or 8 years later...
2nd time: I had a friend whos tank leaked and had to temporarily house his fish with my polleni, i thought it was be okay based on his old tank mate, but the polleni was NOT happy and literally tried to tear the other fish to shreds the minute he was put in, so naturally i took it out and placed it at an LFS i know and trust...
3rd time: My dad decided to get the polleni a friend to hang out with (again thinking back to when he had a tank mate), and again the minute the other fish touched the water he went straight for it and was quite literally ready to kill it, so again i took it out and returned it to the store...
But one thing i've noticed with my polleni at least is, they are very aggressive fish... they will even charge at almost anything, including you if your cleaning the tank... i've actually had my polleni draw blood on 2 or 3 occasions when i wasn't focusing on what i was doing while cleaning his tank...
Beautiful fish, but they definitely come with the behavior to boot in my experience...