Hmmm, good question. I think you have already defeated the non-aggressive theory by adding the snowflake eel. Keep a close eye your damsel, he might be missing one day <img src="graemlins//eek.gif" border="0" alt="[eek]" />
Tangs are pretty good community fish, but add three at a time. Tangs are finiky like that remember odd numbers. I would suggest a Purple Tang, Hippo Tang and either a Yellow Tang or an Achilles Tang. Tangs will also eat all of your filimentus algae if you get any, since they are herbivores
If your are staying Fish only, inverts aren't really a problem. Crabs are know to eat smaller fish once the crab gets bigger. Other than that, you can pretty much add any invert.
Ona side note, some cucumbers when stressed will realease a poisonus toxin into the water that may harm or kill your fish.