"LFS" = Local Fish Shop/Store
I don't know why they would recommend those fish to you either, unless it's because they now have your money, and care more about that than your long-term success. The same may hold for the clown trigger - those guys get big, and can become ... a bit feisty.
Diatoms are a type of algae, often the first to grow in a new tank. They feed on silicates released from sand and stone, as well as nitrate and light. Diatoms and other algaes generally don't show up until after your bacterial cycle has begun to chug along smoothly.
~Bruce