Have to politely disagree. Undergravel filters are old technology. Hard to keep them clean. There are plenty of other, better ways to filter tanks.
But in regards to the OP original question, Fu Manchus will suck up anything it can fit in its mouth, so i would be careful with a 5-line wrasse. They stay small, but are pretty quick, so they might be able to avoid the lion.
Lions are pretty docile (from what I've read, was always too afraid to get one) and can get along with most other big, slow fish (groupers should be fine). Just remember, however, lions and groupers and other large fish are poop machines that need a lot of protein in their diet to stay healthy. What do you currently feed your lions? Don't use feeder goldfish, very poor nutritionally and can introduce disease into a tank.
Flame Angels are beautiful and tend to stay on the smaller side as a dwarf angel.
When you say inverts, are you talking about like anenomes and snails/crabs? Anenomes need a lot of powerful lighting and you should wait a while until the tank is fully established with no problems for about 6 months or so. You'll need snails as part of your cleanup crew, but could become snacks for some of your fish.
You should do a lot of research to make sure what you want is compatible with what you have. Sometimes the hardest thing is just saying "no" to an idea that may not work with your current setup.
Good luck!