Definitely cast your FG through your guides. That's basically why you've gone to the effort of tying a slim knot in the first place.
The topic of having some leader on the spool is always open for debate. Have the leader off the spool and your braid wears from you casting finger, have the leader on the spool and you get more slap up through the guides. I think a popular method is to try and have the leader on only half a spool worth, so that you cast with you finger on the leader but you are minimising the wraps around the spool that cause slapping. Other more experienced people may have a different opinion.