... balance ...

Hmm... Here I'll admit that I feel a little out of my expertise (this isn't really my genre), but what imbalance would result from aliens as equals?

... graphics (the portrait of the planetary governor *must* be a human ) ...

Since this is an empire, not a democracy, the emperor (or someone in the hierarchy of the empire) presumably appoints governors. Thus simply have governors be appointed from within the imperial hierarchy, which is based on the player's homeworld, and governors will (presumably) always be human.

Also I do not want the AI to assimilate player's population.

Why not?

Also your original race is a soft cap to conquest (only terrans can crew your military ships - you can't conquer aliens and then make them into soldiers to conquer even more of their brothers & sisters).

Hmm... I suspect that this could be solved in other ways.

One thing that comes to mind is that inducting a planet into the empire would presumably take manpower--governors, soldiers on the ground, builders, translators, etc--thus reducing your available manpower. This could be countered by conscription from the new planet--but a species that has just been conquered might balk at more than light conscription, and thus be more likely to rebel.

Another idea might be to make the aliens physiologically incompatible with human ships, and their ships (if they have them) be unsuitable for expanding the empire (perhaps lacking faster-than-light capability).

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Yet another thought: it occurs to me that a planet that has just been conquered has likely just had its military crippled; on top of that, its economy and population (and especially that portion of its population trained for combat) may have taken a significant hit. Thus such a planet might not be in a position to immediately offer conscripts for the empire's armies, only slowly recovering--at which point its recovering population growth could simply be modelled with the terran and alien population conflated, and the question of the composition of starship crews might never come up.

Since this is an empire, not a democracy, the emperor (or someone in the hierarchy of the empire) presumably appoints governors. Thus simply have governors be appointed from within the imperial hierarchy, which is based on the player's homeworld, and governors will (presumably) always be human.

"presumably" :) Some players will think (and complain) otherwise :) Generally, I try to avoid such situations where I have to to explain why something works the way it works, especially when there are different interpretations possible... I prefer to show it to the player than to tell.

Also I do not want the AI to assimilate player's population.

Why not?

Balance :D OK, sure I could live with it and make it work and so on but I simply want to avoid it. It does not add to the gameplay and makes things more complex. Also, it's more thematic when you are to "liberate your fellow terrans from the oppressive alien rule", it adds to the mood. Terrans that are not under player's control are supposed to suffer and send desperate letters to the court "Our dear Emperor of all terrans, we beg you to conquer our planet and make us your humble subjects again! We promise to pay taxes in time and serve in your military. Death to the aliens! With regards, Terrans of planet Xefolius XI (temporary under the rule of evil aliens)".

Overall, I could debate & explain what you asked but the truth is, part of the reason is the mood. It's the Empire not the Federation. Terrans are supposed to be special. Aliens as "aliens" fit the mood better.

(plus I don't want to make portarits generator for all possible races :D)

"presumably" Some players will think (and complain) otherwise Generally, I try to avoid such situations where I have to to explain why something works the way it works, especially when there are different interpretations possible... I prefer to show it to the player than to tell.

The "presumably" was on my part, since I don't know your game. You could presumably show how it works, rather than telling. :P

Balance OK, sure I could live with it and make it work and so on but I simply want to avoid it. It does not add to the gameplay and makes things more complex.

Fair enough.

Also, it's more thematic when you are to "liberate your fellow terrans from the oppressive alien rule", it adds to the mood. Terrans that are not under player's control are supposed to suffer and send desperate letters to the court ...

If the aliens are that cruel, is it feasible to have them as citizens at all, whether full or otherwise?

By the way, I may have missed this in a previous thread and wonder whether the answer might affect my thoughts on the topic if discussion: why is the player conquering these planets? I recall something about an incoming extra-galactic threat--is it a matter of increasing production in order to produce a military sufficiently large to meet the threat? Is the goal to protect all life in the galaxy, aliens included? Something else?

For a single species growth rate, it sounds like you want a minimum early growth, increased growth as the population increases, and slowed growth with overcrowding. I would do that in a way that increases gradually, instead of in steps. For example:

growthBoost = 1000

maxPopulation = 1000000

growthPercent=.1;

crowding = currentPopulation / maxPopulation

Then each turn, currentPopulation+= (growthBoost + currentPopulation*growthPercent)*(1-crowding)

With this, growth starts out mostly with the boost of 1000 like you mentioned, and increases gradually until you're mostly seeing the 10% population growth. Then you only start feeling the reduced growth from crowding as your population gets close to the maximum population. If you want stuff like food, infrastructure, and technology, it could affect growth by increasing the maximum population, or the growth percentage.

With aliens, you might use this equation for the total planet population growth, then use similar equations to divvy up the growth between species. As the percentage of upper class increase, it could lower the maximum population allowed for the lower class on that planet, causing them to go beyond their maximum population. If the maximum population is reduced and alien crowding is over 1, they would have a loss in population, making more room for the upper class.