Primitive races
may not need a player slot if they are treated as 'population ruins' as discussed earlier. If they are truly primitive (in respect to other SE4 players - human and AI), then it stands to reason they would not have ships, bases, and support infrastructure AWA not expanding to other planets or systems. So treat them like a ruin...
I personally see them that way - as free slave labor when treated that way. You (an oxygen race possibly), land on a planet with primitive Hydrogen-breathers. You then convince them (with your God-like abilities) to 'be fruitful and multiply' throughout 'the heavens' aided by your 'devine fire-belching chariots'. Of course, this comes at a cost of being totally dedicated to you and your empire...