The real idiot was Balon, for not allying with the only other contender not interested in the territorial integrity of the Seven Kingdoms.
Agreed. Balon did try to align himself with the Lannisters after Blackwater but he would have been smarter to join in with the Starks. One of Robb's mistakes with Balon was arrogantly presuming that he would give him a crown. A usurper going to give another usurper a crown?
I think if Robb had known about Joffrey not being Robert's trueborn heir sooner, he would have joined Stannis Baratheon under his banner. Although if Stannis was stupid enough to try to push the Red God on the North (and he might have), that alliance would have blown up in his face.