Can a minecraft hosted by me be harmful?

Yes, it is harmful as you have open up your PC to public access even there's security setting you can enable, but once you are in public network, hackers are watching you and seach for your loop hole.

Since All network connections are under regular attack from random scripted hacking attempts on commonly open ports, yes it can. I see as many as 2000 attacks per hour from any one script, and as many as 3 or 4 such scripts a day on public server log for machines I maintain. So unless you are confident you know how to protect such a server from hacking attacks think about it. And be aware, "friends" are the easiest way to get hacked. They give out addresses to their "friends", or they decide to try to hack you using some script they find on a supposed hacking sites. They think it is funny to destroy your machine.

Yes, if it is open to the public and running through your computer. If its open to the public the public need someway to connect to your computer and thus could install spyware or other viruses. I would recommend a program like Hamachi that only links the computers in the Hamachi channel.

Try a Dynamic Network Service (DNS). It kinda changes your IP.

If you just want to play with your friends you can try Hamachi. That way it is a more private server