I've never heard a reliable report of a tiger pistol shrimp killing and eating a fish. They don't have that strong of a pop. If you're really worried, get a candycane pistol, they're tiny. Pistols mostly eat worms, amphipods, and detritus, IIRC.
Cleaner shrimp will fight unless they're a mated pair.
Brittle starfish are generally safe as long as you feed them heavily.
Tuxedo urchins are safe with most things, but can scratch acrylic and move small coral frags around.
If you get a white porcelain crab, it needs an anemone to live in, they don't seem to do too well outside them. You could get a green or blue one instead.
Pom pom crabs are very tiny and vulnerable to bullying and vanishing, you may run into some trouble with that and your fish.
Sexy shrimp can pick at corals and often eat zoanthids.
Depends highly on the species of conch. You want a little one.
Invertebrates do produce nitrates, but at a much lower rate than fish. You can't quite add as many as you like, but you can add a good few.
What are your nitrates now, and did you start with dry rock? Sometimes it gives off nitrates.
