If I was in your shoes, I would choose the first option "Start with all 7 ratings". Here is why:

I have noticed an ever increasing disinterest with virtual airlines in the flight simulation community. In my opinion, this can be attributed to the desire by many individuals to have access to whatever aircraft and locations they wish to have access to, and as real life intrudes, another factor being time constraints. Also, in many cases it can be purely from an eventual lack of interest as the new experience becomes routine and limiting.
Regarding the option to progress after a number of hours, I have seen a number of VA groups try to implement that system with such things as:

New pilot, first class of aircraft - 20 hours until promotion (low end of what I have seen)

All the way up to "to qualify for XYZ class, you must have flown over 500 hours with this VA."

Most people, unless they are retired and bored, can not make time commitments like that, even if it is a low number in the 10's or 20's. Also, the airlines that I have seen with the hours system usually have 2 or 3 people on top, able to fly whatever they want, with the other 6 to a dozen or so stuck for months (or years) with first or second class, basic aircraft. Also, the people at the top of those chains are usually just the owners/admins of the VA. Basically, in my opinion it turns a computer game that one would do in their free time into a chore that has to be drudged though in order to do more.

With the thought in mind that I would institute all of the ratings immediately, I would have one condition. Upon initial entrance to the VA, the "pilots" have to take a small quiz on some very basic operating procedures/policies for your airline, and possibly a very small quiz on aeronautical knowledge (IFR procedures, flight theory, etc.)
These would be separate quizzes, with the VA operating procedures/policies one being required. If a user takes the aeronautical knowledge one and struggles, the only thing that would result is a recommendation to meet with another member of the VA or do some other means in order to brush up on some basic knowledge so they are not constantly crashing your VA's aircraft.

Overall, I would recommend allowing your users to use any aircraft in the fleet, and fly any route they choose. A VA that limits is users too much is likely going to become stagnant at the end of a couple months because it turns what was once a fun free-time hobby into a commitment to fulfill someone else's desires, all without getting paid.

Have an active, friendly forum community on your website. Also, host a large number of events that your pilots can participate in, and give a lot of positive feedback to your members. If your forums are dead, there are no events, and the staff is silent, your members will quickly lose interest. This may seem like common sense, but you would be surprised at what is out there!