Nipping:
Because of the behavior you're describing and because most shelter dogs are in their adolescent stage, I'm going to guess you deal with a lot of 1-3 year olds. During these years, dogs transition from puppyhellraisers to teenagemonsters. Not much of a transition Yes, they are nipping and acting like little monsters because of their age and excitement. However, you need to help teach them manners. Nipping is never ever ever ever okay. In dog world, they often communicate by mouthing each other, but, in human world, human skin can not be nipped. It also has a bit to do with bite inhibition, which you can try, but I would suggest teaching impulse control more and rules more.

It's perfectly okay to play fetch with the pups, but do not tolerate them nipping you. If they ever nip you, find an exit and completely take yourself out of the situation. I'm not sure how your shelter is constructed, but if there is a door you can use, go out the door and ignore the dog. Come back in. If dog is still jumping or nipping, walk back out. Repeat until dog keeps all 4 legs on the ground and is showing self-control. At that point, you can reward with a treat or praise or play. In the cases where the dogs don't bring the toy back but nip you instead, I would reward with calm praise because it's clear that they enjoy playing with you more so than with the toy. If you don't have an exit route, cross your arms and face the wall. Do not give the dog any attention whatsoever. Then go about it as above (turn around if dog is calm, keep turning back if dog is being rude).

What I just described is a form of negative punishment (removing something the dog wants to decrease a behavior). Use this liberally!! If you're walking out and the dog is jumping and nipping already, stop and don't move. don't walk forward, don't look at the dog. Wait until it calms down and then proceed walking. If you're about to take the lead off and the dog is mouthing you, stop and stand straight up. Don't move, don't talk, don't look at the dog. Don't proceed until the dog shows some self control. Whenever the dog is being overly enthusiastic to the point of rudeness, take away whatever the dog wants and do not give it back until the dog shows some self-control. It's a very effective, non-physical, non-forceful way of teaching impulse control While you're at it, you can also teach "wait" and Nothing In Life Is Free, aka NILIF.

Re: Rocks. Don't give in. Rocks wreak havoc on a dog's teeth. *sigh* Ivy somehow lost a front tooth and I firmly believe it was the result of chewing on stupid, hard objects (i.e. rocks and raw pine cones).

Re: Grass. A little grass eating isn't harmful. Also, dogs will run their teeth over grass blades to gather scents of who was there. So, you might want to check if the dogs are running their teeth over the blades or actually eating it. And if they're excessively eating it, then you can discourage it, but it's really no big deal in the long run (unless they're eating foxtails! )