There are a lot of "sort ofs" and "maybes" here. You kinda can keep some of this stuff. Kinda can't some other. Here's my take:
1. Clams are pretty and really neat, but like Colin says, expensive, prolly octo-food and quite light-hungry--like corals, despite being animals they depend heavily on photosynthetic symbiotes.
2. Featherduster worms and such seem to be fine. My live rock came with plenty of them. Larger ones might be a bit finicky.
3. Sea urchins are a mixed bag. Rather than a pencil urchin, I highly recommend a blue tuxedo urchin (Mespilia globulus.) Pencil urchins tend to be more like little bulldozers, knocking things over and actively burrowing into your live rock. M. globulus urchins seem much milder, are prettier anyway, have short, relatively safe spines (still, don't try stabbing yourself!) and don't knock things over quite as enthusiastically. In general, they do just fine. Brittle stars are also recommended. Many "normal" starfish (non-brittle) are NOT recommended in a reef tank, however! Many of them tend to eat things you may want to keep, like clams or corals.
4. Corals can be kept. But it depends enormously on the tank, the corals, the other inhabitants, and your willingness to take a bit of financial risk. I keep some corals and had no problems with my first octo. But individuals and species vary--he was so cautious, perhaps he simply didn't encounter them often. Not all corals are the same--effectively all of them sting, but with wildly varying results, and different results for different animals--some don't even notice it at all. At least one of my corals can and has stung me. Soft corals (ones that don't build calcareous skeletons) release toxins into their environment which might, depending on the species, be a threat to an octo, or to you.
Keeping corals with an octo is a compromise and a guessing game. You need to accept the risk that you could lose either animal. If you do your research well and choose milder species (rather than rushing out and buying the Really Pretty Ones, which I've done once or twice!) you may minimize the risk to the octo, but the octo may decide to move the coral, which could injure or kill the coral. I'm not a true expert, but if you want to try corals I might recommend green star polyps to start. They're pretty much harmless to most animals, mostly photosynthetic (you don't really need to feed them,) easy to keep, and they grow nicely.
Tank conditions do matter for corals, but each species has its own needs. Most coralkeepers have stringent water quality, because many of the prettier stony corals require such conditions, but those conditions actually harm some species. In general, you'd want carefully maintained water quality (doable with your setup) and TWICE the lighting you currently have (I understand you have about 110 watts of fluorescent?) That may not be octo-friendly--you would at least want to provide extra dark caves for hiding. You can probably cheat in the lighting department. If you pile your live rock high enough, choose corals that don't demand tons of light, and mount them high on the rock, closer to the lights (say, the upper 1/3 of the tank) then they might be happy with your current light setup.
rusty
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. Given this information i may have to pass on keeping these (at least for a while until i can buy another tank). Haha i may just have to get one of those posters that go on the back wall of the tank that make it look all pretty :P . However, i may go for one of those blue tuxedo urchins. What exactly do u mean by bulldozers? Will it ruin my live rock? Will i have to feed it?
