those should all be okay. The clowns or damsels, or even if they are smaller types of gobies, might be eated, depending on the size ratio of the lion you get and the fish. I would recomend getting the lion when he is realy small. Damsels look great, but i have heard a lot of people complain about how they are realy mean to their other fish. For the most part, it depends on the fish you have though. I have a 20 gallon tank, and in there i have a valentini puffer, a small snowflake eel, an orange spotted sleeper goby, and a small maroon clown. The clown is going to my 40 gallon tonight, and he will be the first fish in there, and once i get a top for the tank, the eel is going in there too. The clwon and the goby are awsome. The goby, because he is always chewing on my substrate, is all my friends favorites. hes relay cool to watch. A clown fish would also make a goob compainion, as long as he isnt too small. I would not recomend a puffer. A valentini are one of the only puffers you can get for a tank taht small, but they are known fin pickers for lions. It all depends on the fish though. But here are some recomendations.
-Some type of clown fish (probably not a maroon since they get 6 inches)
-a sand sifting goby
-damsels and chromis are nice, but may get eaten.
-maybe a dwarf angel like a coral beuty?
-the bangai cardnal fish i hear are awsome, and i noticed you like them.
-maybe a bicolor blennie? I hear nothing but good thigns about them
-firefish
-royal gramma?
These are jsut some suggestoins of fish that can fit in your tank. I am obviously not saying to get them all. Just some ideas so you can pick out fish you like....And a sump is a good idea.
by the way, way to go on picking everything out before you buy, thats nothing i was not able to do when i started this hoby, although im still a newbie. I jsut cruise these forums all the time.