the easy way to open the chocobo ruins caves :
once you have clasko in the chocobo ranch and can catch chocobos which gysahl greens in battles just catch them anytime you come across one.
in chapter five go to his ranch and go to each chocobo, in chapter five the chocobo will now show how high level wise it can get. Make sure you have four chocobo who can get to level 5 and make them your runners (if not dismiss until you have space for four more and go and catch some) p.s chocobos will be least likely to runaway if you have a full compliment of 14 in the chocobo ranch.
once you have four runners who can all reach level five, preferably all bold ones. then send them out to either calm lands or djose road, go out to calm lands and fight seven battles come back SAVE YOUR GAME, and talk to clasko as long as each chocobo comes back and HAS SUCCESFULLY FOUND TREASURE you can then level them up to the next level, rinse and repeat until all are at level five send them out one final time fight seven battles and go back into chocobo ranch and you will get a scene heralding the opening of the ruins.
p.s if you come back after fighting 7 times and find a chocobo has runaway soft reset the game, as chocobos running away is decided pretty much randomly so in another playthrough they might not runaway at all.