Corals is where it gets a bit more dollar-heavy, mostly because of the lights. If you want mushrooms, and soft (leather-type) corals, they can use less exotic (but still potent) lights. If you want clams or the stony corals, you're into metal halide lights and their ballasts. For the softies you might do all right with your pump; for stonies, again, you'd want to get a sump, bigger pump(submersible) and skimmer, replacing your filter with reliance on your live rock. You might want to start with that step: a 10 gallon sump would set you up fine, with an Urchin skimmer, then dump the filter and just let it do its thing. Clowns will host with a few of the stonies like frogspawn---what the frogspawn makes of the bargain, I'm not sure. Or even with some leathers, like toadstools. I would advise you against an anemone if you want to do corals: anemones have a habit of moving, they're messy when they demise, and they end up causing disasters to the whole tank when touched off into a fatal stinging-war with corals.
You'll step up to corals with some (2-3) mushrooms, likely, tolerant of most lighting conditions and reactive to mistakes, but not catastropic too easily. You'll need test kits. The halloween hermit I don't know, but most of the big ones are not coral friendly. The tiny scarlets, about the size of the end of your little finger, are reef-safe. Your feather duster will be fine in all circumstances.
At this point you have to begin thinking about next-steps: "Will I go to the stonies?" "Will I go toward the softies?" That's a branching point of equipment and tank dedication, because softies and stonies don't get along easily, and though most people sneak a few softies in, you have to be very careful about positioning them in the current, to be sure there are no conflicts. Don't go anywhere beyond mushrooms without thinking about that.
One thing that helps defray expenses of this hobby is trading frags of softies or stonies with each other---because corals grow, some of them pretty fast, under good conditions, and if you watch where you put things, you can break them off and swap with others to add to your variety---and help excuse the flow of dollars from the wallet.
HTH.