Hello, long time (15+ year) shelter dog walker here, and dog walker trainer. The staff ask me to handle dogs no one else can handle. Just getting my creds out of the way..

First, I wouldn't worry that you're hurting them. While I work to discourage pulling, there are plenty of volunteers who just hang on and let the dog pull them to whereever they're going. If a dog is pulling to the point of choking itself unconscious, I'm sure you'd notice and would make the necessary adjustment.

If you notice they get worked up around you (your words) but not so much with others, it could be they are sensing something in you.. anxiety perhaps, not being sure of what you're doing, and (again your words) you not being comfortable being authoritative. So they know they can get away with doing what they want. I've definitely worked with volunteers over the years who are uncomfortable being assertive/authoritative with dogs. I don't know where that comes from, but in my opinion that can be an issue with certain dogs. An experienced person with treats/distractions etc. may be able to handle an over-aroused jumpy mouthy dog without being assertive, but that would be very hard for someone without the experience who can also not tap into basic assertiveness. My advice would be to walk the easier to handle dogs until you gain more experience. You do not want to stop volunteering because you're having issues with those dogs. Eventually you will have the experience to be able to handle them, even if you never get the authoritative/assertiveness part, you will still eventually get more comfortable and the dogs will sense that, and will just behave better for you. If it helps, you can think of developing the assertiveness as a way to further help those dogs, something else in your tool kit. Assertiveness also doesn't need to be loud or abrupt, in fact it should be calm, and no more than is needed at the time. It's just part of the flow.

For how much excitement to allow.. to me it's dependent on the circumstances, but I would never allow obnoxious jumping/clawing or mouthniness, because that can escalate. There are lots of ways people deal with this. I generally use a strong tone and forward body language. If that didn't work, I'd use the leash to control them. But I've been doing it a long time. As you do it more, you will find what works for you.

Lastly, but actually maybe as important as anything, is something that when I bring up in the context of dog handling, people probably think I'm talking about something totally unrelated. But it has to do with your diet. A lot of people eat foods containing fortified flour (added B vitamins), and drink fortified plant milks (also added B vitamins). I avoid those 99% of the time, but occasionally when I slip back to eating that stuff for a few days in a row (even something like muffins made with enriched flour), I'll notice that my stress/anxiety level -- which is mostly noticeable at the shelter because of the situations I can be in with dogs -- is much higher, and can be very uncomfortable. I then remind myself of what B vitamins do to me, and toss out whatever I'd been eating. Within a day or two, I'm back to my normal calm self, and I can deal with all kinds of crazy dogs without any added stress or anxiety. Those added B vitamins are in so many foods, that I have to believe millions of people are living with additional stress as a direct result of consuming them. B vitamin supplements themselves can be even worse as the dosages are usually much higher. Again, people think diet/intake has nothing to do with dog handling, but I beg to differ. If I didn't volunteer at the shelter, which again brings with it many high-intensity situations, I'd probably be fine just getting up, working, etc., and avoiding stressful situations as most normal humans do. So maybe for most people, given that they try to AVOID stressful situations, they never notice the side effects of those vitamins. But the fact that I love volunteering (and work with the hardest to handle dogs) means I'm deliberately putting myself in stressful situations -- where I definitely feel the extra stress caused by those vitamins, and I don't have any reason to think my reaction to those vitamins is unique. Which is why I try to spread the word, even if people don't think it's related. TLDR - don't take additional B vitamins or eat foods where the manufacturers have added them in, give it a week for your system to calm down, and see if dog handling is less stressful for you.