I've setup several frag and general coral tanks for a reef shop about 10 years ago that were basically banks of 4 40 longs. The bottom 40 was the sump with a refugium and the three 40s above all shared the same sump. They worked out surprisingly well. A shared sump system sounds complicated but in practice it's the simplest system I've tried. It let's you get away with having to deal with only one large skimmer, one mechanical filter sock, one set of chemical filtration, one volume of tank water to test and dose, etc... You get to spend a lot more time handling corals and less time on everything else. I'm so glad you decided to go this rout.
I will say that quarantining new fish becomes non optional if you want to avoid illness. Most illnesses will spread to all the systems with just one infected fish. That may not matter if you aren't planning to add fish to the other tanks. If you don't add fish you won't believe the amount of pods
you'll see in the frag tanks! It gets pretty crazy.