I have kept 2 vittatus and 2 goliath with very little success.
They are only really aggressive towards other torpedo shaped fish, I often find they are the ones to get beaten up or killed. They spook easy and often die from either knocking themselves about too much or from jumping out. They are also very sensitive to water conditions, you have to keep it super clean and no drastic changes in quality. I switched out the filter on my tank with my most recent GATF and he died over night.
They wont bother your rays, but if you want to keep them with gars, arowana or other similar fish assume that the hydrocynus will kill the other fish or stress out because of the presence of the other fish.
I truly don't think they are great aquarium fish, they should really be kept in schools if they are kept, they seem much more relaxed in a school. When not in a school it seems like overtime you turn the lights on they smash around the tank aimlessly hitting everything in their path. Furthermore they should be kept in massive tanks from the get go. I'm going to try one more time with H. Forskahlii, which will probably come in between 4"-6" like most hydrocynus and I will start it off in a 6 foot 130 gallon by it's self and move it into something much larger once it hit 10".
Most of them start off feeding on live food and usually will switch to pellets or frozen food in time. However it's important not to starve them especially at a young age. They die from starvation very quick. I find the youngsters are messy eaters too which makes it hard to keep the tank clean. They go around biting things in half and leaving the rest behind. So it's important to have good filtration and I found using a whole bunch of tiny fish, like small tetras are a good a way to avoid this issue. That way they swallow them whole instead of taking bite sized chunks and leaving the rest.
They come from fast flowing waters that are high in oxygen so provide lots of flow and good water agitation.
On that note some people do keep them very successfully, I think the trick is getting them through the baby stages. They settle down once they are a bit bigger. Vittatus are easily the better choice. I always had much more success with them then goliath.
Good Luck,
JMorash