so I wanted to start running the game, but wanted to start with single level runs and currently there isn't a category for solely Cairo. I was wondering if another level category for the intro could be made?

Since Cairo is so short it doesn't really warrant a speedrun.com leaderboard. The closest thing we have are the community gold sheets, which you can find at slygolds.com though this only shows the best segment recorded for Cairo. A leeaderboard would prove quite stale when timed in whole-seconds.

As a general aside, learning Cairo first generally isn't recommended since it is the only level with tough major skips via clipping/out-of-bounds movement. I highly recommend starting out on Episode runs, particularly Episode 1 is the easiest and simplest to get into.