Hello Fishymom, it will depend on what kind of lion you are thinking about adding to your tank. A lion may be fine with the Puffer, it will depend on the individual Puffer, but keep a eye out for the puffer nipping at the lions fins in the long run.
What works in a tank today may not tomorrow, many of the fish we get are juveniles and can coexist at the moment, it is when they start becoming adults or have become an adult that things can go terribly wrong and they will no longer coexist.
Your clown may eventually become food no matter what lion you add. A volitan will grow fast, extremely fast, they should reach 12 - 15 inches in around 18 months. Though lions are not aggressive they are predators and will prey on fish that are smaller, lions can eat around 1/2 their body size. With that said, the problem comes in when a lion such as the Volitan outgrows their tankmate.
For your tank you may want to look at some sort of trigger, such as the BlueThroat, Crosshatch, Black Durgeon and even the Pinktail, these trigger seem to do well with lions and not try to pick them apart in the long run.
Maybe a wrasse, my personal favorite is a Bird Wrasse, or maybe a butterfly fish.
Hope this helps
Kaye
