The Humu Humu should be okay with a lion. I'd double check this one if I were you though.
Yes, Mantis shrimp will eat small inverts and fish. They really should be a species only tank, however, I have seen them in tanks with very large fish. Don't know if that's the wisest thing to do, but I've seen it done.....
Even though lions are considered aggressive fish. They're really quite docile. They'll just eat anything that they can get in their mouths. They're not really suited to be around other aggressive fish....
In a 72G Fowlr, semi-aggressive or aggressive tank. You'd be better off with considering some of these:
Trigger (Humu Humu, Picasso, Pink Tail, etc) but only one
Dwarf Angel (a large angel is too big for a 72G)
Snowflake eel??? Maybe
Hawkfish
Damsels (evil little fish!)
Female Maroon Clownfish
There are some aggressive/semi-aggressive Wrasses as well and some have some amazing colors to them.
Foxface - they're fairly peaceful, but they're quills have venom.