I doubt the Lionfish would try to eat the cuttlefish at the size it is. However I would not recommend the combination because the cuttle might try to eat the lionfish, and that would not end up good. Another thing is cuttlefish have sensitive skin so if the lionfish got angry and decided to go into sting mode, the cuttle would stand less of a chance than a regular fish.
I do not recommend you purchase a flamboyant cuttlefish. From my math, your tank is roughly 150-180 US gallons. I could be a little off, but I think that's about ballpark. Flamboyant's maximum size is 8 cm, which equals to between two and three inches. Unless you plan a group, a 3 inch maximum for virtually the only inhabitant in that big of a tank is in my opinion a waste of space. You can try out a group, though you need to remember that they can and do cannibalize and you may not end up with what you started with. Cannibalism can usually be avoided if they are raised from a young age together, fed non-cephy foods (no squid), or have a large tank- which would be your case. I still would not put it past them to at least fight.
Next is toxicity. I did read somewhere that Flamboyants are believed to be toxic producing something similar to TTX. I don't believe that any further research has been done on the topic, so it is not even known for sure if they are toxic or not.
Finally, like Pea-brain mentioned, the wild populations are not known of this species. It is in a similar boat as mimic and wonderpus octopuses. Though they are offered (usually at an extremely high price, low chance of survival, and being collected toward the end of their lives anyway), most people do not find it ethical to buy one and it is frowned upon by most cephalopod keepers as well. If Australia is getting them in- and people are buying them- it is just a matter of time until they make their way to America, where there will be a bigger demand and more casual keepers that don't know the first thing about keeping this animal wanting to get one. Also, and this is directed toward Pea-brain, breeders have to get their animals from somewhere, so just waiting around for one to try to breed these guys could take forever. Then you aren't even guaranteed that they will be successful and there is disappointment.
Anywho... I would go with a different species than M. Pfefferi. S. Bandensis is a great one to go with. They stay small as well so you would want a group. A bigger cuttle should do fine in your tank too though, like S. Officinalis, S. Pharaonis, or S. Apama. I do not know of availability over in Australia, but I would guess that at least S. Apama should be able to be special ordered.
Hope I helped.
