At the end of the day, most people prefer to stick with the basic units instead of getting fun with it (at least in quick games). But it's still a good idea to be creative anyway. There's always the good old fashioned copy-paste method, where you have human infantry with the normal stats then dwarven infantry with the exact same stats and so on. That way players won't have to account for any weird stat shenanigans when playing quick, while still getting to see a variety of different species fighting together. Problem with that is the slider when divvying up a stack to hit a bunch of places. Don't overdo it is what I'm saying. What do you think? It's a big map, so it makes sense to have a big selection. 8 races total? Something like that sounds good to me, 'cus any more than that and it's hard to keep track.
Nothing's stopping you from adding variation in the units though. Some players really like that. Would it be a good idea to stick to a handful of main races, and in secluded areas create a new "branch" of one of them? There can be your standard dwarves or something, but nearby that volcano-lookin' thing you can find hobbits as well. That way there are two "layers" - you can divide the map into x amount of chunks that have a specific main race on them, and then every now and again there's a country that allows you to recruit a little something unique on top of it.