I don't know how big Kirby is, our Yoda used to do the same thing with the cat harnesses we used for him (the simple ones we got from Petco). We were given a step-in harness (here's a picture of one:
http://www.moderntails.com/store/cus...at=277&page=1), which helped greatly , as it's too far down there so that he couldn't get his mouth down there to chew on it.
If Kirby is 6 to 8 months, he may still be teething now, so anything he can get his little teethies on, he'll chew up! If Kirby is home when you are not there, you may want to consider removing the harness, as he may get his chin stuck when you are not there, which would be pretty sad...
As far as the poop is concerned, try to keep a very close out on him while out on walks and try not letting him near the poop with either a stern "No!" or as others have mentioned "Leave it!" when he gets near. You can try bribing him with a small treat right after the command which may help him learn it better. But yeah, poop is gross, but it seems EVERY Yorkie initially likes smelling and eating it, so you're not alone there. It took Yoda a bit to stay away from poop, but I think after a year or a year and half (he's now almost four), he was pretty good and able to stay away from poop on his own. Another thing you could do with poop is spray some Bitter Apple spray on it, that has worked for some, as that changes the smell of poop to something most Yorkies may not like.
Oh yeah, there's nothing wrong at all with being overprotective, I've been an overprotective Dad since we got Yoda! Never wrong to be as safe as possible!