You have to offer any tankmates plenty of hiding spots where your Ps can't go.
Our pleco has spots where he can sit in PLAIN VIEW and my piranha can't touch him. Simply because there's not enough room for Whip to get vertical to try and nip at Lucky.
ALSO, when I put my pleco in, I took Whiplash out, and shifted some of the plants around, breaking up his territory. Whip tried for about 5 minutes to pester the pleco, and now only headbutts him when Lucky's in his way.
Not every tank mate will be eaten, otherwise the animals would be isolated in the wild. I know, there's more space, but the fish do branch out during the wet season. Which would mean, they'd be the only species left if they ate EVERYTHING they came across. But, as I've said, don't get too attached to your ps tankmates, they'll probably end up being dinner one day.