Micro brittles are neat. They sit in holes and stick their legs out... If you have a lot, you'll end up with little patches of what looks like thick hair. They're great because they'll get into really tiny spaces to clean for you.
The others are usually harmless... My LFS has those in a display tank. You usually see maybe six-ten of the little things just crawling around, they don't get to epidemic proportions unless you're seriously overfeeding. Those little guys are remarkably good at not dying... I saw one get bit in half by a fish, and the non-eaten bit just crawled off. They don't feel pain and don't have brains... Plus, any tiny bit of the center disk can grow back. If you cut one into five even pieces, all the pieces with center disk would regrow.
I hate hearing people say "it's only a $3/$5/$1 fish/shrimp, so it's ok if it dies, I can just get another." It's still an animal! All animals should be treated like they're worth $10,000.
29 sw: Damsel, shrimpgoby, pistol shrimp, waspfish
65 fw: Rummies, glowlight tetras, pencilfish, darters, ottos, f betta, goby, dwarf gourami, ninjas
29 fw: Chili rasboras, pygmy cories, P. Gertrudae