Am I the only person who thinks the ponies will just love and tolerate the shit out of Aku? And which ponies would succeed at this? I personally think it would either be Fluttershy or Twilight.
And what would if Aku met Discord?

I don't think Aku will befriend the ponies but rather try to destroy them. Since Aku is literary evil and his life's purpose is to destroy the plague of hope. But Aku would likely play along then back stab them when he gets the chance. If Aku was randomly sent into equestria, the first thing he'd do is destroy, break, and rule.
If Aku met Discord before he was reformed, Aku would make a deal with discord to eliminate Samurai Jack forever. If it was reformed discord, he would be more than annoyed.