English speakers, especially in Australia and North America, will say "there's security cameras". You should probably not use any contractions for "there is" or "there are" when writing, unless you are quoting speech. But in common spoken usage, you will often hear "there's" as a contraction for both "there is" and there are". According to the Oxford English Grammar: “This usage is common in informal speech.” In other words, in spoken English, you can get away with "there's" for a plural object. In written English, avoid contractions generally, unless quoting actual speech.

You are correct. For plural (more than one of something), it should be "There are ______." For singular (one of something), you should say "There is ______."

You're right. It should be "there are".