I've got a predator tank and a mixed reef, I wouldn't share the sump between them (more convenient for maintenance, but if one crashes they both do, but also, my pred tank won't sustain sps, softies do great in there, but the water is too dirty for sps)
As for stocking, lionfish are great, I've got a fuzzy dwarf in mine, but once you get into things like sea goblins, it greatly limits what else you can get (mostly because most fish will just get eaten). Even my fuzzy (he's still small at 4 or 5") eats fish if he can catch him (even though hes really well fed).
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