a) step in harnesses can physical damage to large dogs... if they are pullers. If he becomes a puller a body harness will not be a good choice.
b) I would not switch him to a head harness unless his walking is a problem, especially if he is as shy and uncomfortable as you make him sound. Most dogs DO hate head harnesses, but some walk beautifully on them when they have a chance to get used to them. Some dogs get so stressed they shut down completely when a head harness is put on them. So it really has to be gauged on a dog by dog basis. As for head harness brands, the only one I recommend is by Newtrix.
http://newtrix.ca/docs/index.php It leads from behind the head, unlike others, and puts less tension directly on the muzzle area during a correction. The head harnesses that lead from beneath the muzzle aren't as good as they tend to wrap uncomfortably, and jerks the head in a very unnatural way that can actually injure. Also, it puts the leash and head harness in prime chewing territory. And as Bronwyn pointed out, some dogs hate the head harness so much that they'll walk beautifully to not have it put on!
c) any trainer who sells every dog on one training aide is a bad trainer. No trainer should believe in ONLY martingales, or choke chains, or pinch collars or head harnesses. A good trainer will believe in using different aides on different dogs. I have no problem with choke chains on the proper dog, (for instance a dog who generally walks well on a loose lead but occasionally needs a small, quick correction) but martingales are preferred. Also, the trainer I work with charges $185 for a 7 week course (1 hour classes). There is also a maximum of 6 students per class. Some places have more classes that are shorter or less classes that are longer, and some will have huge classes with lots of "helpers" but no one-on-one time with the trainer for you... so do lots of shopping around. You should be able to visit facilities, sit in on classes and be able to talk to each trainer about their training philosophy. Stay far away from Petsmart classes :lol:
d) as for high protein being bad for a puppy... :roll: tell that to the raw feeders out there. But any food changing you do should be very slow. He's going through a lot of changes and being as sensitive as he is, he'll need everything done slowly and carefully.
e) Javakittie said it well.. Not everyone in animal care can be a vet, not everyone can be a groomer. I am living proof of this. I can do the basic stuff but I in no way claim to be a groomer. I make sure my customers at work know this when they ask things of me sometimes. If anything, make sure you get yourself comfortable with nail trimming and ear cleaning, and that he's good about brush-outs. If he'll behave while you brush him head to toe, you can probably get away with less trips to the groomer. I really put emphasis on the ear cleaning though. All the poodles that come to my daycare have ear issues.
f)If he likes liver and gizzards, make sure to always have some around! You'll be doing lots and lots of training for the next little while! It sounds like he's adjusting really well and that you are really enjoying him.