For the hidden espers i was wondering does it give you a certain time limit to get them b4 u move on further in the story?

No.

Rephrase the question. I do not understand it.

I think he's asking if you get to Bahamut, for example, then are you still able to get all the espers. The answer is that you can get any esper anytime. There is no time limit to getting the espers.

You can't get Bahamut.

Still not understanding. Unless he means you need to defeat an esper within a certain time limit like Zalera, then yes.

I think what they are asking is if they proceed past a certain point in the game do some of the hidden Espers become no longer obtainable.

I think he's asking if certain Espers are permanently missable.

Well, correct me if I am wrong but if you sell the Great Serpentskin then you can't get into Barheim Passage and you can't get Zalera. Right? Or is there another way in?

And Harkus he meant Bahamut the PLACE. Duh.

I think he's asking if certain Espers are permanently missable.

Well, correct me if I am wrong but if you sell the Great Serpentskin then you can't get into Barheim Passage and you can't get Zalera. Right? Or is there another way in?

And Harkus he meant Bahamut the PLACE. Duh.

Oh yeah, my bad.

Well, correct me if I am wrong but if you sell the Great Serpentskin then you can't get into Barheim Passage and you can't get Zalera. Right? Or is there another way in?

You don't actually need the Serpentskin to complete the quest (and thus receive the key): only the minimum amounts of the items she asks for are needed. The Great Serpentskin is only necessary if you want to get the best reward from the patient.