Leathers generally tend to be more toxic than SPS do. I only have a couple of small leathers in my SPS tank. You can get around this by running carbon to filter the toxins out of the water. If you are going to run carbon then its important to make carbon replacement an important part of your tank maintenance. I have a seperate tank for large leathers, other softies, and a few LPS.

This was demonstrated to me recently when I messed up and pointed a power head at a very large toadstool in my tank. A large open brain immediately began to recede and look very bad. The toadstool was irritated and started to excrete toxins because of the high flow. After running carbon , doing a couple of big water changes, and removing the power head the open brain began to improve. It now looks as good or a little better than it did before.

Unfortunately there are no tried and true rules for mixing corals. They are all specific to the a particular species. I have a huge Galaxea in my tank that most people would tell you that con't go into an SPS tank because of its sweepers. I have it in a corner of the tank with a power head blowing on it to keep the sweepers away from nearby SPS. I think that you can do a lot of things that you aren't supposed to be able to do if you put money and time into designing your tank to maximize water quality.