Hello. Ribbon eels have a very poor survival history in captivity. They usually are very difficult to feed and extreme escape artist. If you are not very experienced marine fish keeper, I would try an easier eel at first. Maybe a Snowflake. Also the other fish you listed. The flounder needs an very open tank, with little rock, which the eel you want would need. Also most flounders get rather large. As far as the Stone fish. They are extremely venomous and will eat very large fish.
I would highly recommend getting a good protein skimmer. The fish you listed will produce copious amounts of waste and though your eheims are okay, they will not remove excess nutrients like a good skimmer wood. i would also add some LR in order to raise the beneficial bacteria level in your display. Tim