Terrell specializes in Fire and Dragon type Pokemon because he's the leader of the Magma Colosseum and Dragon type Pokemon are often associated with Fire type, as a stereotypical dragon can breathe fire. The reason his outfit looks like Groudon is that Groudon, according to a few Pokedex entries, despite only being Ground type, has heat powers. (Or however you'd describe it.) Also, Terrell is the leader of the Magma Colosseum. Magma is molten rock, rocks are from the ground.