It never hurts to have multiple races for variety, but when using the same races, you never have to worry about balancing issues. It's all bout choice and personal preference, I guess. Depends on scenerio, too. On my AoS map, I used all the same races because it takes place during a civil war. -VGsatomi
To make them look different, simply take two races and edit the units / create new ones that are equal to each other, but use different models. E.g. a knight and as counter-part a knight with Abomination-model. That's how I do it.