I'd guess Galleom was mainly chosen because he wanted a robot boss for some Classic mode runs. It's one of the most common bosses (Well, I didn't count, but it was the impression I had while playing through with everyone).
I figure they went with Galleom because they wanted a more brawl-distinctive boss, & maybe to referene the original stuff, and he fit more than Tabuu, a final boss, and the swarmed master hand bosses which are a bit different. He has a mroe conventioanl boss fight than Duon, so maybe that's why he won out over that one?
As for bosses, you're mostly right.
Ignoring the Master hand, the "special" bosses like giant palutena or dk, & Master + Crazy fights (& not accounting for PP & Joker) its
Galleom at 6
Dracula, Rathalos and Ganon at 4 each
Marx with 3
Giga Bowser & solo Crazy Hand are tied for 2
Giga Bowser is bizarrely underutilized