I've had assassins for some time (and a lot, at LEAST 40+ that I can actually count). They have never ever eaten anything other fish/shrimp that were not already dead. They did eat my first batch of eggs from my honey gourami though. I've had them with platies, guppies, neons, honey gourami, cherry shrimp and amano shrimp... no troubles at all.
They eat whatever food they can get ahold of, flakes, bloodworms, algae wafers, etc. Seeing as I started with 6 and have had 70 at highest count (sold some to the LFS this summer)... they are thriving on normal tropical fish diet. LOL