I would not recommend any sea urchin for a reef tank simply because they are just too good at what they do, your live rock can easily become base rock with an urchin around. That and they get fist sized and would most likely not find enough to eat in a 30 gal tank.
The multi-colored slugs are probably nudibranch and should never be bought as they are highly specialized feeders and little is known of each species diets. Those diets that are known include sponges, corals, tunicates and such. The odds of you having their specific needs in your tank is highly unlikely.
I actually had a tuxedo urchin for years. However, at the time, my tank was very well established and had a very fast coralline algal growth rate. That single urchin wasn't able to even put a dent in it in my circumstances-- but that won't apply to every tank. Another issue is that they can dislodge corals that aren't carefully affixed.
Lettuce Nudi's and Sea hares are generall ok. I have a lettuce nudi, and I love him. He's survived a terrible maiming too thanks to a powerhead, tough little guy.
The Sea Hares can ink though I believe, but its not toxic like octo ink ircc, someone correct me if I am wrong. Beyond that I think all nudi's are a nono...
Sea Hares CAN be toxic just like some cucumbers can be when startled. You'd have to do the research to find out which kinds are/aren't though...I don't know. Most nudibranchs can be toxic if they die in tank but I don't think the lettuce is.
the only slugs i have seen up here are the multi colored ones......very beautiful slugs but i was told they dont do anything helpful and are a pain to feed....
i was looing at the lettuce slugs on the net awhile back.....very interesting lil fellas.....
If you are looking for other things to add - feather dusters are beautiful and relatively easy. Porcelain crabs are filter feeders and are beautiful - will not harm corals. Adding the plant halimeda is a pretty way to help keep nitrates down and is very pretty in the tank. Hope that helps.
That's ashame...I didn't realize that the CBS would go after it. You never know for sure with these things. I had a peppermint shrimp - supposedly reef safe - kept eating my corals. Just returned him today to the LFS. Then I have all sorts of sponges that aren't supposed to live in small tanks that are doing great! Looking forward to seeing what you end up stocking with.