Speed is irrelevant in Grand Prix, so I use the Koopas in the Paratroopa car, Toad and Toadette in the *original* (thank goodness) racing kart, Mario and Luigi in the Red Fire for the mediums, and sometimes Mario and King Boo in Wario's car. You know, if you use Wario's car but not him and he happens to be racing with you, he'll be driving his same car. So, there are duplicates.
"If you throw an object in the air, its shadow gets bigger. So, if you put a baseball next to the sun, its shadow would block out the Earth." ~One of my Friends