Had an vieja syn with my endli. Pretty difficult to feed because the vieja started to love the taste of massivore. Ended up having to add a little more each time for the endli, and the syn has a huge stomach lol. No aggression issues. Although I have a vieja argentea about 2in that love to constantly attack my 10in endlis tail so I have to rehome the argentea.
I kept my first two Poly's, both sens, in a cichlid community for five years with no issues. I gave away most of the cichlids when I moved, but kept the Poly's and a single convict that no one wanted and put them in an aqua scaped 40g with plenty of hiding spots. The sens were at around 9-10" at this point, the convict in the 4" range. I added an angelicus syndontis to the mix. That single convict terrorized the polys and the angelicus day in and day out, nipping, chasing, boxing out of food piles. I eventually found a home for the convict and set up a 180 gallon poly community.
I dont have aggressive cichlids but have the challenge of getting food to the bottom feeders like the weeksii and del. They would have zero chance to eat if i did not feed at night. I put a mix of floating and sinking pellets in of many mixed sizes but the cichlids make quick work of it. I would have to seriously overfeed to get food down to the polys with my regular feedings. feeding after lights go out has been perfect.
I do not notice any aggression toward my two poly but then I have pretty mild cichlids which include a pearsei, regani, heterospilus and red bay snook.
I have had many cichlids with polys. I had red bay, Trimac, dovii, psittacus, oscar, red devil etc. i have my dovii and psittacus with over 20 polys in my tank. they just do fine... somtimes when the dovii is very moody he would pick them up but the bichirs wouldnt thrash lol. as for food. i never worry lol
With my polys I've successfully kept RD's, oscars, salvinis, green terrors, blue acaras, severums, peacock cichlids, victorian flamebacks, JD's, and probably others I've forgotten.
The only issue I had was with a single, ornery mayan cichlid. He doesn't tolerate ANYBODY though. I'd say it depends on the individual a lot of the time.