Hello and welcome. Biggest recommendation is to read a good sw beginners book. Research research and more research. To touch on your questions tho. A 55 is ok but not ideal for sw. A canister filter is not reccomend. A hob is a better option. No you do not need a skimmer. You will if you want...
Imforbis is spot on w a qt. Look into a dwarf angel.
Dwarf lions are one of my favorite fish. I like zebra myself. Ghost shrimp are usually needed at first then they will wean on to frozen foods. Shrimp, krill, ect
Only get a pair of clowns. Nix the clown trigger. There great looking but after awhile it will be all you have left. Look into a niger, blue throat, instead. Personally i like a zebra eel there safest eel with most fish.