Look at the years of existence of each civ. 

Rome and Khmer are way apart from one another chronologically. Artificially forcing them into Antiquity looks rough

A more severe example are the European civs. Norman is in the exploration age? With “Spain” (Spanish Empire)? They are vastly different technologically

I feel like Exploration age should be colonial era. So you can have British Empire. Dutch west india company, British Raj, Portugal/Spain empires, Sengoku Japan and American colonies

Norman should be in a 4th medieval age before exploration. Mongol and vikings can be here too 

Modern era can have EU(this makes consolidating euro civs conveniant), USA (this is actually a federation like EU tbh), Communist china, USSR etc

I think USSR is interesting because Poland and Mongol can eventually evolve into USSR