As cephalo said, MoM, MoO2, and Civ 4 are structured very differently with regards to how different races are handled. I'll outline them here to get everyone on the same page.
In Master of Magic, races are handled at the city level. You choose a race for yourself at the start of the game, but it has no (or very minor?) effect beyond choosing the race for your starting city. The race of each city determines what buildings and units can be constructed, the race of the population that lives there, and any bonuses the city receives (pop growth, gold production, etc.). When capturing a city, you have a choice of capturing it and dealing with the unhappiness caused by the city having a different race type from your own or razing it to the ground and replacing the city with one of your own race (I can't quite remember how much you had to destroy to switch the race type).
In MoO2, races are handled at the citizen level. The race you choose at the start of the game confers two types of bonuses, global and citizen. Global bonuses are things like "faster ships" or "better diplomacy", citizen bonuses are things like "extra production" or "better researchers". Each population point (a citizen) belongs to a specific race, these citizens can be moved around among your planets as you wish. Citizen bonuses are applied to each citizen of that race, regardless of the player that is currently controlling them or what world they are on. Growth of all race types on a given world is tracked individually. Citizens of a different race that yours have a slight penalty and mixing multiple races on a given world has a more significant penalty. I don't remember what options you are given when conquering a world, I think it was a choice of wiping out the population or not. Regardless of your choice, it would take several(many?) turns after conquering a world before it was fully productive for your empire.
In Civ 4, races (nations) are handled at the player level. You choose your nation at the start of the game, and that determines the capabilities of all the cities in your empire. When capturing an enemy city, you might choose to raze it for strategic reasons, but when you keep it, all of its capabilities will be of your nationality (buildings and units that can be produced, bonuses it receives, etc). The only effect of having citizens of another nationality is to increase unrest if you go to war with their original civilization.