If you want to try Ojijen, introduce it slowly as you did with the ZiwiPeak. Orijen is a huge step up from James Wellbeloved in terms of quality and nutrition, it is as good as it gets as far as kibble goes.
You can feed ZiwiPeak exclusively if it suits your dog, some people find it a bit rich so prefer to feed it alongside a quality kibble or fresh raw.
There is no need to feed the same food all the time, this is just a marketing ploy by dog food manufacturers. Variety is key for optimal nutrition, even with dogs that don't adjust well to change offering different protein sources within the same brand is a good idea.
So as long as your dog's tummy can handle it, and you introduce any new foods gradually, feeding several different foods is a great idea. It can help to keep your dog interested in his meals too, after all who would want to eat the exact same thing day in day out?
Glad your boy loved the Ziwi, it is such a great product. Mine love it too, it is the only commercial dog food I feed. I feed raw, and they won't touch kibble now, but go loopy for a square of Ziwi.