I am a first time Octopus owner. The Octopus is in the tank with several other small animals, which I assume will eventually be eaten by it.

I have heard the octopus will leave a small pile of it's leftover prey bits after it eats a few times.

Is this true? If it is, how much is left over?

In the wild, they make a "midden" of piles of crab skeletons and the like, frequently. In tanks, it's good to get those cleaned up so they don't ruin the water quality...

That sounds like a good idea. I was more asking because there are several fish, 10 snails, a hermit crab, and a starfish in the tank, and sometimes I can't tell if the snails are even in there. If it ate one of the fish (which seems to be missing), will I see it's remains cast out?

Mine has a little graveyard of fiddler appendages that build up each week until I clean

That sounds like a good idea. I was more asking because there are several fish, 10 snails, a hermit crab, and a starfish in the tank, and sometimes I can't tell if the snails are even in there. If it ate one of the fish (which seems to be missing), will I see it's remains cast out?

I'd say it probably depends on the size of your octo and the size of its appetite. You will more than likely just find a dead fish with a couple of strips of meat torn off near or inside the den. I don't think any smaller octos are going to pick a fish skeleton clean. I could be wrong though. There aren't too many people on here that keep fish with octos, food or otherwise due to the risk involved, so it's a hard question for some of us to answer.

What risks are involved?

Attacking/harassing the octopus is the primary concern with tankmates. There have been reports of fish picking octopuses eyes out. Really it's going to depend on the particular octopuses disposition and the fishes disposition. Some A. aculeatus have been kept in mixed tanks with passive, peaceful fish without incident. It seems many other octo species common in the hobby like to eat some fish, so keeping them in a community tank is usually a gamble.

For the sake of being economical, most people use damsels as both a way to cycle a tank and then to be future feeder fish for the octopus or whatever other predator they put in their tank. Unfortunately damsels although relatively cheap, are also very aggressive, and often enough found to be a challenge for an octopus to catch. While I understand the purpose of adding fish to the diet, and also the thrill of watching the octopus hunt, there are safer ways of going about it.

Also, the presence of any fish that harass the octopus will cause stress and likely make the octopus go into seclusion, possibly die prematurely.

To get the most out of the octopus it's just best to provide a controlled environment. The moment you introduce a fish, that fish has the power to do as it pleases. If it turns out to be a bad fish you have to hope you are around when the fight goes down so you don't come home to an armless octopus or a dead octopus. Removing a fish from a tank with sufficient live rock is a whole 'nother story. That's going to freak the octopus out even more...

There are just so many scenarios. As the old saying goes, "better safe than sorry!"

rotten fish!