
A standard 100 won’t really give you much more to work with than a standard 75 considering they’re both 4’ tanks, I said an 80 is better than a standard 75 because my version of the 80 adds an extra 2” in width, going from the 18” width of the standard 75 to 20” so hopefully the extra 2” would be a bit better for the aquascaping. What my version of the 80 “sacrifices” compared to the standard 75 is about an inch in height but IMO height’s not nearly as important as Length and width.
My version of the 100 gives 60” or 5’ of swimming space over the 4’ of the standard 100, while adding a further 2” in width compared to my version of the 80. 18” height IMO would make it a shallow tank but again height’s not all that important IMO unless you’re keeping some larger and taller fish.
It depends on what risks you’re willing to take with corals, no angel or butterfly I’d say is 100% guaranteed reef safe but the Genicanthus angels and some of the butterflies I mentioned are about as close as it gets. If I were you I’d be willing to accept that risk for what are some really nice looking fish.
Another route to go, if you are willing to get a nice and tight fitting top, would be quite a few wrasses. Evolved has pretty much a wrasse only tank. Many Fairy Wrasses, pretty much all Flasher Wrasses, and all the smaller Halichoeres would be fine for a 100. However in terms of fish families that would mean less variety.
If you see a couple of species that interest you feel free to keep us updated.