The Elite homeworld was mostly desert. As of my knowledge, there was very little grassland there at all. To keep this more accurate with the Halo story, I think you should change the setting.
And the Elite race has very very very poor technology. Their native weaponry consists of wood/stone spears and whips and to my knowledge, no vehicles what so ever. Depending on what time period this map is being placed in should determine the entire set up (weapons, vehicles, ground texture). Also you might want to find out what their planet was like. How many moons they had, etc, just to keep it accurate. I'll look up some more information for you and post back later.
EDIT: I suggest you take a good look at
this page because it has some in depth information on the Elite culture. It has everything you will need to know (well, at least most of what you'll need to know) about Elites. This may influence your map a bit.