Can a country have both unity like the Romans and diversity if yes how if no why not?​

Yes i do believe they can. Indonesia, for example, their motto is Bhinekka Tunggal Ika (Unity in Diversity). Despite all the differences in its multi cultural society, there is a sense of unity among all people there. this is maintained by respecting others choices and their backgrounds and allowing interactions without questioning someone’s religious, social or economic standing.