Mystic your experiences don't equal hard fact. Just because mixing a knife with dats didn't work for you doesn't mean it will never work. There are many many people keeping knife fish (and all manner of things) with large datnoids, hell the probably even cross paths in habitat. In this case it didn't work out, possibly because of a big size difference between the knife and the dats, which is why I suggested Bjorns CK as its 40cm+ and already living happily with a couple of large datnoids (unless he's sold them already).
People always try to define fishkeeping with rules, XX inches of fish per gallon, must have 10x turnover for filtration, can't mix this with that, etc etc. These are not set in concrete, in fact there are usually so many exceptions its a stretch to even call them rough guidelines. What works for one person might not work for another, all you can do is try it and be prepared to pick up the pieces when it doesn't work.