The topic on using unravelling spells to counter other spells raised a general question about fast-casting with me:

  1. I have already acted this round(due to a higher initiative roll) and cast a spell. Can I still use a fast-cast defense against another spell (which is cast at the lower initiative of some other dude)? -> this would in fact give me the chance to easily cast more than one spell per round.

  2. If I used fast-casting to protect myself, can I cast without penalties when my turn in the initiative order comes up again?

And on another note:
3) A knight is charging me (no spells involved), can I use a fast-cast 'kill an animal' spell to stop the knight?
3b) If four knights are charging me, can I fast-kill their horses as long as I make the Qui+Fin vs. the knights initiative? (taking a -6 on the second roll, -12 and -18 on knight three and four)

Fast casting lets you react to another person casting a spell. So yes, you could cast your own spell first, and once he gets around to casting, you could cast a counter. In fact you could cast several, if the opportunity rises, but with increasing difficulty.

Yup, you can cast as many spells per round as your skill allows for, as long as they are all "Fast casts". Which means they need to either be Spontaneous spells or Formulaic spells you have mastered and taken the "Fast Cast" option for.

I'm not sure if you can cast a fast-cast spell, and a non fast-cast spell in the same round.

David Chart suggested in another thread, if I recall this correctly, that you can, but it pre-empts your action in the next round. That is, a really quick magus can cast a spell, then respond to a slower magus's spell, but he does not get to cast another spell of his own in the next round, because he used that action to fast-cast.

Your memory sounds correct...

Taking a look at HOH:S page 21 (right box) with Paris Sophia, it states:

This seems to cover the issue, I think.