Well first, I would make sure all the horses have their own grooming supplies, because if you find that one has a bad skin condition, if that horse is sharing grooming tools, it might infect the other horses. Then curry out most of the dirt and hair, bath them with shampoo, then use conditioner everywhere ecept the saddle area. Then while they are still wet, you can spray some show sheen or vetrolin shine(I personally like the later) and then let them dry. If the horse seems nervous and unsure, I sing to them while bathing them. They always know where you are and most horses enjoy it!
Oh, and for tangled manes and tails, NEVER GIVE UP! My horse had a matted spot on her tail 6 inches wide and 3 feet long, and my mom got it out! IT CAN BE DONE! Good luck and give us some before and after pics!