Rome began diverse, and thrived from its conquest of Ostia circa 400 BC to the final destruction of the remnant of the eastern empire by the Turks in 1453 AD.
Wrong! The peoples you talk about were all Latins speaking a variant of old Roman Latin. The non Latin speaking people were the Etruscan’s which Rome promptly set about conquering. The Roman was initially very exclusive-see Cato the Elder and the Greek ambassadors. Diversity is linked to the loss of Republican freedom with the rise of Imperial slavery and finally collapse.