[Kalimac]

Legolas probably did fight in the Battle of the Five Armies (since Tolkien never states anything about him in "The Hobbit" we are free to speculate, but it's reasonable that he would have accompanied his father). But although the Elves and Dwarves became allies, that doesn't mean that they stopped sniping at each other; think of WWI and the English and French. They were on the same side but it didn't stop them from indulging in less-than-complimentary descriptions of each other in the decades afterwards.

As for recognizing Bilbo - of course, in the Elvenking's palace Bilbo was invisible 99.44% of the time, so chances are high that Legolas never laid eyes on him. As for recognizing Gloin - well, for one thing it's been 77 years since they last saw each other, and Gloin's probably changed a bit. Another thing - the Dwarves were bundled straight from the audience with Thranduil into their prison cells, and left only by the cellar. If Legolas didn't happen to be at the audience, he would never have seen them close up. Even if he had been at the audience, it's understandable that he might not remember the face of every one of Thorin's followers.

Battle of the Five Armies: Again, Thorin was the main Dwarf with whom the Elves were treating, so it's unlikely that he ever saw Gloin close up. As for Bilbo - well, we can only assume that Legolas just didn't happen to be in the same tent with Thranduil that night when Bilbo came slipping in with the Arkenstone. A hobbit would, after all, be pretty memorable.