What speakers will fit?

I have a 1997 Buick LeSaber Custom with concert sound II, i already have the 6x9's for the back and the 5.25 for the doors, my question is, what sizes tweeters can i put in place of the old tweeters in the door, size?, also i am going to add in the 4x6's in the dash, but due to the mounting dept with the ac vent in the way inside the holes, what will fit, will 4x6 plate speakers fit? because plate speakers is what i really want to put in there.