Trade will indeed be hardest.

Ad population growth: Like in EU (and most grand strategy games) population growth should be calculated relatively, not in absolute numbers. There should be a base population growth modified by different aspects: religion, health, immigration, emigration, war, food supplies, etc... And there should be a base maximum for the population in provinces, again modified by production technology, land-fertility, maybe even magic, etc...

The population groups seem like nice ideas, but they don't only influence army composition and manpower, but also income, productivity and miscellaneous economical behavior. And here we should think of general traits which can then be applied randomly (frog-like fire breathing bards who eat brains wouldn't necessarily fit with the CK-specific groups )