I wouldnt keep a lion and a trigger together. From what i have herd the trigger will rip the lions fins. If your lion is not a dwarf lion then i hope u plan on upgrading. Volitan lions need a larger tank then a 55 but should be fine for a little while. If you want to get the puffer get a small one like a valentini puffer. If you want another lion you can get a fuzzy dwarf, a fu man chu, A dwarf zebra (my favorite), an antennata, or a radiata. Many to choose from but i wouldnt get more then 2 lions. Four fish is enough and i hope you have really good filtration because they are really messy eaters.
So lets review, No trigger
Upgrade in the future
Good filtration
Four fish is plenty