An introduction by a newbie to the community is highly acceptable. It is always a good way to inform the community your intention for being on the forum.
Sure, but this comes with an important caveat:
Introductory threads: If you feel the need to post an introduction thread please take the effort to write something substantial or don't bother at all. Threads and post that contain only one word/sentence such as "hello & welcome to the forum" will be removed without warning as it causes unnecessary clutter.
People normally wouldn't care who you are or what your background is, especially in this community where privacy and anonymity are emphasized. It's probably only truly acceptable if it's mentioned in passing along with actual questions/topics you want to discuss. Everyone already knows you're a newbie from your post count/forum rank.