Rokushiki is a set of moves that utilize the raw power of the body in specialized tricks that increase combat potential. However the fundament of the Rokushiki is that it is only as strong as the user.
Since we are given an arbitrary 500 douriki as the minimum required to be superhuman and we pair that with Lucci's comment that 'one must master all 6 to be superhuman' we can say that that is the number
required to utilize all six techniques. However we also are given the fact that Nero who is not superhuman has master 4 of the 6, such that numbers below 500 are certainly valid for at least some of the techniques.
Futhermore we know that the 6 methods become more powerful the more physically powerful you are. As evidenced by Lucci's reaction to Franky's less than spectacular attempt to do him harm.
So lets look at it this way. Each of the six methods has a minimum level that is required to execute it and for each point you are above the minimum the abilty gets stronger.
NOw this is an example. If my douriki is 500 I get tekkai which gives me a defense of 100 an arbitrary number. For each 2 points of douriki above 500 I will get an additional 1 point of arbitrary defense score. So Fukuro at 800 has a arbitrary defese scoreof 250. Impressive, but Lucci at 400 is sitting on a defense of 1850, attack that would kill Fukuro woulnd't even move lucci, his arbitrary defense being literally 7.4 times stronger.
Now just because tekkai improves at arbitrary rate X with douriki Y doesn't mean that all the other skills improve at the same rate or require the same douriki to use.
Lets take Soru, lets say that Soru requires arbitrary douriki number 200 to use, once you use it you move at arbitrary speed number of 100 and for every 20 points of douriki you move 1 more arbitrary speed number faster. Nero might have arbitrary douriki of 400 meaning he moves at arbitrary speed number of 110. Faster but not all that much faster than the basic soru. Fukuro with his 800 would be moving at arbitrary speed number 130. Faster again than nero but not all that much faster. Finally Lucci with his 4000 is moving at 290 arbitrary speed numbers. Thats only 2.2 times faster than Fukuro as opposed to the 7.4 times harder.
So the point being that the most important part of each individual Rokushiki being different, either learning the technique or how powerful you are when you execute it.
Either way its a method that requires a certain level of superhuman body to use. Thus no matter which way you wish to play it if you don't have the power you have to get off the field. Thus Garps training for luffy would grant him an incredibly high douriki, somewhere in the several thousands but since he wasn't around in the later parts he never taught him the tricks of that powers application.
Thus luffy migh be sitting on arbitrary Douriki 3500, enough to bare handed spar with Lucci and certainly enough to use soru and all the rokushiki if he was so trained, but only knowing the application of soru that is all he can use.
Logically Luffy should be able to use the Soru in his normal form as there is absolutely no way that he could fight evenly with Base lucci who as 4000 if Nero has less that 500 and can use the soru and gets utterly demolished by a casual luffy.
Gear 2 could increase the power of luffy by double or triple but even without that boost there is no way he would lack the superhuman body needed to use Soru.
To continue onward from thi point we could imagine a number of scenarios in which the interaction of Douriki and devils fruits or Gears combine to create physical plateaus.