I think if he's happy with the harness when walking, then that's fine. Ideally you want to get two fingers underneath there (round his ribcage) to make sure he's comfortable and not too restricted. Could you make another hole with a pin in the harness - kind of somewhere in between?
In terms of the poo eating, this very issue came up at the puppy socialisation class I went to and the behaviouralist there cited a case of a dog who was doing it excessively and making itself so ill that they had to re-train it. She said it was because it had to have 'a duty' or 'a responsibility' when it was out in the park. So instead, they used lots of retrieving and carrying games to divert him. I think he was a real hard case though but it was an interesting insight into why they do it
I've googled some pages for you. All this sort of stuff is very interesting. You never stop learning with a Yorkie!
According to some of the sites I've run though, one of the things you should check with your vet is if your dog has any internal parasites that are causing unusual parasites. You could also add courgette to his diet because for reasons unknown to researchers, it puts them off theirs and other dogs' faeces. I don't know what you call courgette in America but I think it might be zucchini (long, green plant)
This site is just general information on coprophagia
http://www.isabellevets.co.uk/health...ophagiadog.htm
Another one
http://www.understandinganimals.com/article/3
Another that says the issue should be addressed in a number of ways
http://canines.com/library/solutions/coprophagia.shtml
This is a very informative site
http://www.paw-rescue.org/PAW/PETTIP...PoopEating.php
This site recommends teaching him the 'leave it' command
http://www.celiahaddon.co.uk/pet%20p.../dogs/poo.html
So, looks like you have some work on your hands but it's a really resolvabe issue so don't worry too much!
Annie