Knowledge of Merry and Pippin
 

Merry and Pippin were captured by Ugluk and the fighting Uruk-Hai before anyone had knowledge of Sarumans army. However, when they were with treebeard, they seemed to know all about Sarumans army and Rohan. Is there something I overlooked, or anything I should understand?

People did already have knoledge of Saruman's army. Gandalf had seen it being created during his imprisonment on Orthanc and Eomer, Theodred, and other leaders of the Roharrim had had scermishes with them on several occations. Merry and Pippen were most likely just retelling what they had heard as the reason the Fellowship could not take the Gap of Rohan and had to instead try braving the Red Horn pass and eventually the Mines of Moria.

Also in Rivendell they had learned quite alot.

Yes, after Gandalf told about Sarumans treachery to the council of elrond, Im sure Frodo must have told them, or anyone else once they joined the fellowship.They could have over heard quite a few things on the journey, noone really got much privacy, I would think.

Yah, they would of overheard a lot on the road (though they seem quite clueless till they meet Treebeard). I doubt they would have heard anything in Rivendell. They weren't at the Council of Elrond if I remember corectly (but they were in the movie).