I'm no expert I can only give you my experiences and I've only kept bichirs sparingly and only Laps and Endli's. As others have said stay away from anything bite sized. I myself have been surprised at how capable predators they are despite coming off as rather slow moving in general.
Some cats in the Pimelodus and more peaceful Mystus can become disturbed by them. I don't know what does it, but my Ornatus, Blochii and M. Wolffii get pretty skittish around them; even small individuals. Other more robust cats seem to pay them no mind and vice versa.
The only other issue I see with some cats is on one of the issues I've had with cichlids as well, which is competition for food. As I mentioned before Polys can be slow, even when it comes to food. As such I've had problems making sure the Poly gets enough to eat when in a tank with cichlids and catfish. I believe it's an issue others have encountered as well at least with cichlids. Other then that IME cichlids leave polys alone for the most part.
As for Plecos, well you've heard some have had issues. To minimize chances of problems I'd make sure to try and stay away from omnivorous and carnivorous Plecos. There are strictly vegetarian plecos and you maybe safer comm'ing them.