There's no better way to learn then by gathering as many personal experiences as possible and I definitely would like to hear more about tankmates as well TheeEmotionalPayara. I actually didn't know about any of the tankmates other then the Pike TBTB till I saw TEP's response to you. After I did a search of TEP's posts and it seems as though the tankmates of the Apu are a Pink Tail Chalceus, Flagtail Prochilodus, Pleco and a Brycon not to mention again the Pike. Is that correct TEP? If so I can understand why some of it works. The Brycon, Chalceus and Flagtail probably rarely if ever go into the strata the Apu inhabits, combine that with their serious speed and agility they would be a hard if not impossible target for the Apu except maybe under the cover of night. Still a well fed Apu probably barely patrols the bottom of the tank never mind the upper water strata at night. As for the Pleco, I'm guessin' who would really want to eat one of those as it'd be like eating a fishy pinecone so the Apu probably pays him no mind, probably. The Pike in my opinion is the biggest surprise as a working tankmate. Between their aggression and the Apu's crankiness and them inhabiting the same water strata for the most part, I'm surprised there's no trouble.
TBTB, I'd be glad to share my own experience with these guys so far. As you know I had one who had gotten to about 12 inches before his demise. I got him around the 6 inch mark and he was co-hab'ed with an Oscar 6 inch and a Red Devil 5 inch. The Devil never bothered the O but for some reason had a hate on for the Apu. The Apu basically didn't bother anyone at the 6 inch mark and was harassed by the Devil. The Apu grew quickly to 12 inches out pacing the O and the Devil and at the 12 inch mark ate the Red Devil but left the O alone. The O and he co-hab'ed with no problems after that despite the Apu being large enough to eat the O. Once I moved the Apu to a bigger tank with 3 stingrays(2 Retics and a Marble Motoro) he for some reason tried to prey on them and successfully ate 2 stingrays and was killed by the third who also died from the Apu's attack.
Fast foward to now, I've got a 3 inch Apu comm'ed with my Tarantsy who range from 1.5 inches to 4.5 inches as well as with Geophagus Abalios 4 inches and Red Spot Severums 5 inches. When I forgot to feed him he has preyed on two of the small Tarantsy. Other then that he keeps to himself wedged behind a UV filter and has to be handfed Silversides(hate that he hasn't taken pellets for me yet). I did though get to see him in action, he hid upside down under a rock that made up the roof of a cave I made and all I could see was the reflection off his eyes. When anything got close he pushed them away with an open mouth doing no damage till the smaller Tarantsy approached at which point he pounced and devoured them. Presently he's passed up Tarantsy that I feel he could eat, preferring only the smallest of them. Maybe that boldness happens with age or he actually thinks of the bigger Tarantsy as tankmates.
Thinking of getting a jelly cat my self 