Ryan,
Your right, Lions can be kept with other fish, but the simple fact is your tempting fate. Lion fish are predators, and while your feeding them prepared food has probably diminished the desire and need to hunt (I mean hey, you can swim with sharks right, even better if they have had a meal before hand.) you may one day find yourself wondering where a few of your invertebrates have gone, or a small fish... Lions can be kept in tanks with more than other lions, it's just that a lot of people choose to keep them alone, not only because they are the hoover of the reef, but they are simply stunning and can be appreciated on the own.
Oh... and the size of a fish is not the only important factor, it's speed and ability to hide is probably the tantamount trait to look for in co-habitating it with a lion.
One other thing... one of my friends recently added a young lion to his tank, he had used a blue wrasse to cycle the tank... well.. the wrasse is no longer part of the community... fast, and, bite sized... granted, that lion isn't trained to take prepared food yet.. but you get the idea... it works for you, great... but it doesn't work for everyone.
BTW, what foods are you feeding?