I think Helu is the most talented back in Washington. However, he lacks the ability to move a pile which will hurt his TD numbers. He's very explosive though and thus doesn't need a huge amount of carries to be fantasy relevant. He also gets added bonus in PPR leagues.
Royster is a less explosive, but more powerful version of Helu. He figures to be at the least, the best goal line back.
I wouldn't touch Hightower. He's a decent back, but coming off injury with 2 capable backs in front of him.
Shanahan has recently developed a bad reputation for his RB situation and admittedly most of it's deserved. But on the other hand, he does usually stick with his guy in each game. If you look over the course of last season, it was usually 1 guy getting 65-75% of the carries in each game. Just not the same guy week in and out. That said, I don't think it's that much of a headache to grab Helu and Royster, both of whom are relatively cheap, and play whoever starts the game. It's just something you'd have to stay on top of and make sure you're starting the right guy and if in doubt have a capable safety net to start instead.