Similarities through history.

[Tellos Athenaios]

But the repubblic was multicultural, of course not like Carthago (but only because Carthago was older) who were the romans? they were local italians!
Rome had also been dominated by the etruschians, so it is naive to say the "roman race" because there were no roman race.

Carthage is an entirely different league. A different world, pehaps. Carthage relies first and foremost on it's commercial activities - Rome on it's military. Hence, Carthage cannot afford to be anything but multicultural - by depending on trade, you depend on a relatively open, and multicultural society. You need it, because you rely on foreign relations to trade with to provide for the tax you need.

Race isn't entirely correct, we should speak of ethnicity. And in such sense, the Romans did understand the concept. Gens, anyone?

Roman citicens of course they will always have more right than other non romans, but that is the same what happens in modern time, a american citicen (or Danish, Italian, Rumenian or whatever) will always be able to do more than who doesent have the citicenship, like voting and this means a citicen of a state will always have more rights. Rome needed more people to work and fight for them....and here we have the plebs. Who were poor local people, but also many individual (not migration) who settled in rome or around rome.

The plebs is basically consists of everyon who are:
1) Roman (citizen)
2) Not a patrician.

And the Plebs mostly consisted of voting citizens - however that hardly meant anything in Rome. You were expected to vote as your Patronus wished. If anything being Patronus or Cliens mattered a great deal more than being a member of the Plebs or a Patrician. (Though the Patricians were de facto always Patroni.) To a certain extent, you may compare Roman citizenship with being member of a the maffia. Being the Pater familias of your Gens can be compared to being a Godfather.

You cannot compare modern day citizenship with Roman citizenship: those are two entirely different species. Roman citizenship meant protection from the law, especially against non-Romans. Non-Romans couldn't rely on such protection.

About the multicultural of the empire, well rome builded a empire above the nations so the multicultural element was actually the core of the empire, otherwise it would not have survived for so long time.

Now that's a contradictio in terminis. Romans created their empire by Romanizing their subjects. That's got more to do with Star Trek than with Multicultural Society. Anyway: it was a matter of removing the previous culture in favour of the Roman one. The one big exception to this was, of course, Hellenic culture. Here, Roman & Hellenic culture met and adapted to each other.