#1
you may or may not remember a thread similar to this, but, whatever. I can kindly make up solos, but should they be more high pitched or low pitched?? In case you were wondering, I play stuff like Megadeth, Metallica, Black Sabbath type music.
#2
Both?

Go from high to low, but high solos will come through in songs better generally.
#3
Do whatever sounds good in that particular part of the song. Variation is always good so feel free to spice things up a bit, you don't want all of your solos to sound the same.(*cough*Dragonforce*cough*)

You shouldn't really have a set plan for doing solos, for example writing a song and just deciding it will have a high and fast solo in it without trying anything else.