The sevs will be fine. I have lots of sevs and polys together and never had even the smallest problem with the mix. But i`d never put polys in with a flowerhorn and certainatly not a flowerhorn with sevs.. Sevs may well be cichlids but they`re peacefull, not far from angels and discus really, and the flowerhorn will grow to be a torrorizer of that tank if you mix them.
Just my opinion though...