See for example: Mapping groups of constant values
Sure you can - the r tag is very common on Stack Overflow for example.
One of the "risks" in having high reputation is creating tags by mistake - so it's our responsibility as high rep members to overview our questions after posting them and correct such mistakes.
Removing the tag from the question will make it disappear after a day or so assuming no other question was tagged with it during this time, so no harm done.
Yes, you can create a tag with a single letter, tag c ,r and d also has one letter. Creating a tag requires 1500 reputation. In your case I assume that it happened because you were in hurry to post a question. You just hit the 'Enter Key' after the tag constants and during that time a key of 'l' has been pressed. Now you have reputation of 3K on Stack Overflow so system has created a new tag l