No, you can't. But, if the plural form represents a single idea, then you can use "it's". Example: (To have) that many people is not a good idea.

No "it's" = it is. Example: It is a small cat. "there's" = there is Examples: There is a dog[ one dog] at the gate. There are two dogs at the gate [ plural]

No, not at all. However, I often catch myself doing so when I'm not paying attention. Very annoying. The proper verb contraction for plurals is 're (are): you're, we're, there're... However, it is much better to write the words fully.

Not correct, it's sloppy English. Even we don't speak it perfectly.

I've heard people saying sentences like "it's seven girls" so I was wondering if it's correct.