People might avoid putting Lionfish in a Predator tank because Lions aren't really aggressive. The general rule is to keep aggressive fish with other aggressives because they can defend themselves and to keep peaceful fish with other peaceful fish. The Lions are sort of in between, like Anglers, because they can suck in anything small enough to fit in their mouths, but they aren't truly aggressive fish. A Lion with Angels or Triggers or Puffers is defenseless because its natural venom spines are easily avoided by aggressive fish. You can hope to spot signs of aggression before the Lion is damaged to the point of dying or killed, but there is no guarantee.
The general rule when adding fish is to put the least aggressive in first so yes a Lion should go in first in a predator tank.