The expression 'the Regina Georges of the world' means 'Regina George and all the other people who are similar to her'. We use this device when we are using a person's name to indicate a particular stereotype. 'The Regina Georges of this world' is a way of referring to all the people who behave in this way - Regina typifies, and is representative of, this type of behaviour. We use a definite article because it's a particular group of people. As for the apostrophe, it shouldn't be there - it should just be an 's' to indicate a plural.

"Regina George" is normally a proper noun, but by implying that there are multiple Reginas, "Regina George" is essentially treated like a normal noun i.e. you can have "a Regina George" or "a hundred Regina Georges". I don't know the context, but I'm guessing a Regina is meant to be someone who acts like the real Regina George. The apostrophe is a mistake. The second sentence is very awkward. I doubt it was written by a native speaker.

1) "the" is proposing that there are lots of RG type people in the world RG's is just bad English. It is called the greengrocers apostrophe so you can see "potato's" 2) is just bad English, though the "a" may just make the song scan.