True optional level starts at level 8--at least that's how our coaches explain it. They say level 7 is partly compulsory as well.
Skipping levels (or more accurately--competing one meet at a level to "score out") really depends on your skill level. If you have the skills for the current level, then it isn't that difficult to score a high enough all-around at a meet to more up a level. If you want to do that, you need to talk to your own coaches though as not all gyms allow it. They should know you're interested in moving up as soon as you can and help you train to reach that goal.