If you are going to turn that 90 into a reef stick with the dwarf, much easier to work around in the future. Although I have rarely seen a volitans over 12", you should plan on 15", so will probably find your 90g to be too small eventually. Lions with lots of space will use lots of space, and vice versa. Their sedentary behavior, from which many derive proof that a large tank is not needed, is misleading.
Plus the feedings can keep water quality far from optimal.
Plus lions don't really appreciate the bright 'reef lights'. It might even damage their vision, which is pretty poor to start with.
Shrimps will be a meal for the volitans, but larger cleaners and coral bandeds will probably be ok with the dwarfs.
And btw, lionfish do not eat corals. They just poop on them.
hth