I´m with Sloanjh on this. What is your problem with changing the required rank?
Sure, Steve could write a lengthy program to let Aurora judge a design of yours by the tonnage, speed, armor, shields, numbers and strength of armament, size of magazins,... but would it be worth the effort?
Per default, unarmed, commercial ships without survey sensors require rank 1, survey(?) and jumpships require rank 2 and armed ships rank 3. This is good enough for me.
If I design small ships (as Sloanjh says) lowering the needed rank is not only possible but absolutely reasonable.
What rank did the "captain" of a PT boat or a Flower class corvette in WW 2 have?
I highly doubt, those ships were commanded by full Captains.