1) What they told you about mixing tangs is partially true. It's true that tangs don't tend to like each other and may fight but if done correctly (i.e., putting the less aggressive types in first or everyone at the same time, moving the rocks around when you add the next, etc.) it can be done. You have the best chance of success if the tangs don't look alike. But I'm afraid I come down on the same side as others here, you're about maxed out on fish already.
2) What is the mandarin eating? They require a mature tank (and you said yours is pretty new) with loads of liverock or they will usually starve to death slowly.
3) Wean your lionfish off of freshwater feeders right away! It's common to feed saltwater fish rosys or small comets and it doesn't SEEM to harm them until years down the road. Feeding freshwater fish to a saltwater fish causes deposits on the liver and is a good way to introduce illnesses to your whole tank or various parasites to your lion. He would do better eating thawed out frozen chunks of seafood such as krill, clam, fish meat, etc.