~ You generally want the sun behind you, the photographer. Having the sun behind the subject can work as well but, depending on where the sun is, the photograph can easily turn into a silhouette (cool technique but not ALL the time, lol).
Really though, there's not a set "IT MUST BE THIS WAY" thing. Different times of day = different techniques.
~ For me, lighting depends on the color of the horse and the effect you're hoping to achieve. In general, you always want there to be enough light. Different aperture settings on your camera will help you when it's too bright/dark but a faster/slower aperture will effect the quality of your photographs.
Generally, probably the "worst" time to take good photos is when the sun is really overhead. The sun being overhead usually results in a lot of glare.
My "favorite" time to take pictures is a couple hours before sunset when the sun is about halfway down, for light colored horses. I just got a bay and I'm needing to re-figure out my whole photography "scheme". It seems as though whenever my gray mare looks great, the bay mare looks weird and vice versa.
But other people have other favorites.
~Professional-type stances? Probably Googling "halter Arabian" and "halter Quarter Horse", in the pictures, will get you a sufficient answer.
Flash, I would not use it. Personally, I feel like the flash is something that really should not be used as much as it is. Some professional-type cameras are made to be used with flash but flash in your average picture is not helpful - it washes things out, makes nice editing impossible, etc. In some circumstances, a flash can be useful but those instances are rare - in my opinion.
Play around with it though, you might find that you are skilled with your flash and you like how it looks. Who knows! :)