Here is why:
Your planned set already has Dragon-type coverage in a move that has more power. You could add Toxic, like you mentioned, or maybe even Hypnosis, some move that would add some more utility to this set.Kingdra and Kyurem can be handled by some other teammates like Lucario
I think the only Ice/Dragon-type is Kyurem and its other forms. Kyurem is the only form allowed in UU, so that can be dealt with using Lucario
Kingdra is the only Water/Dragon-type allowed in UU, so you shouldn't have much trouble if you are using a Hydreigon on your team.
Hope I helped!