it's true that paramicia do normally have a weakness, but it's the paramecia that are more wide spred in fuction eg; Gomu gomu no mi can be used in a number of ways, inflation, like a slingshot, recoil attacks. then there's gears 2nd as well but a inu inu no mi turns you into a dog or hybrid onthing more (discounting rumble ball).
Luffy's a good fighter, but even he struggled to be a certain leopard Zoan. Plus, I doubt the previous Gomu Gomu no Mi users would have mde as much use of their DF like Luffy does with it. Luffy's a trained fighter, creative, reckless, brave… And on top of it, Gomu didn't give Luffy physical strength it just made his body rubber and the abilitiy to stretch out limbs. So Luffy's exploitation of his DF was certainly a factor in his victory over Rob Lucci, however he also has an incrediable will power (Amazaon Arc - Haki).
In other words, the Gomu Gomu no Mi, did not souly win that fight for him and had he had the Gomu Gomu no Mi, but lacked the factors in that fight, that Leopard Zoan would have been all over him and any other Gomu Gomu no Mi may have been floored.
None of CP9's Paramecia's put up as great a fight as the Zoans, as I said before, the Paramecia are just useful gimmicks and since their a "catch all" group I can't even say their 100% certain to be useful... We cannot judge the usefulness of one against another too soundly.
Also Kaku, who just ate his DF, still proved a more challenging fight to Zoro then soap woman to Nami was. Once the soap was removed, she had only the fighters actaul strength to worry about. The most notable factor in the two ladies fight was simply the soap bubbles weakened Nami and all that DFs power virtaully went away once Nami figured it out. And lets not forget, aside from beating every Zoan down on top of it, the other other way of beating a Zoan so far is to get them water bound.
The best example of a Zoan Vs Paramecia with the Paramecia would be Wapol Vs. Dalton... But we had interferance from Wapol's men which spoils this example. Dalton clearly won his stike on Wapol, but Wapol exploited Dalton's concern for his people and plus Wapol had the 20 doctors on his side.
Other then that... Pell was alarmed by Robin's powers yet didn't know how to fight them. His concern for Vivi and anger at BWs would never have won him that fight. Though its not too big a factor since compared with someone like Zoro Pell is nothing anyway. It suprises me how often Robin gets away with her powers without too much hassle and the only one to avoid her submission was Moria and that was because of his shadow tricks.
There isn't too many Zoans s fr so we've not seen too many fights involving them which is a shame. Regardless of position their still just as worthless against a Logia since in the end it levels out.