Dee Dee,
these are lovely but the lighting definitely challenged you. For the posed portraits, I'd use my 580 flash for fill. It would mitigate the dappled shadows and even out the lighting overall.

With the 580, there's not much to figure out. Set it for high-speed synch and let it do the work.

You could use a reflector but you'd need an assistant to make sure it was focusing the light on the subjects. I agree that the studio lights, while they'd help with the lighting, would probably spook the horse.

I love the standing portrait!

Jim