One thing we really didnt get was when to place the pig. We both waited until our last go and put them with our best possible farmer, not sure if that is right though.
It's not wrong
Definitely the pig needs to go with the best farmer, and which one that is will not become clear until late in the game. Also, playing the pig too early when you're using the dragon on towers could mean that it just gets returned to you and you've wasted a move.
In my experience, though, the biggest farm is clear some time before the end, and is often the only farm to be completed. Other players want to be a part of it, and the pig can be your way of improving a score on a farm which several people score - if everyone scores a farm, then effecively no-one scores anything, and that's when the pig is perhaps most useful. So my advice is: leave it late, but not
too late!