The syno is already fat enough to avoid a full size del or whatever I think and it is still growing. The knives are slowly growing but at 4" and 5" they are safe IMO, and they are getting better at procuring food.Never kept a poly with a knife honestly, you don't want anything slim with a poly though (Like a dojo loach or anything). Synodontis catfish are generally considered pretty safe since they're pretty heavy bodied. However, you're looking at getting an upper jaw... they're not as proficient at murdering fish since their mouth doesn't really have the capacity to snatch anything even close to their circumference. Honestly depending on the size of the knife I'd be more concerned with it bullying the poly than vice-versa
They are african knives, they don't bully anyone any more than pushing them outa the way like when the salvini is eating where they want to eat, I don't think any issues will arrive there. there is a lot of wood spots and some pipes as well for hiding already