I'll get into choosing a vet in a moment, but to answer your other questions, you are doing absolutely nothing wrong by upgrading the quality of your kitten's diet. In fact, you're doing the absolutely RIGHT thing, and don't let ANY vet tell you differently.
As far as bathing is concerned, I have only bathed a cat when it was medically necessary, or when they got into something that would be dangerous for them to ingest while bathing themselves, like motor oil. Under normal circumstances, cats keep themselves quite clean. Bathing can strip the coat of essential oils and dry the skin. I prefer to maintain a healthy, shiny coat through proper nutrition and hydration.
Now, about the vet ... You should DEFINITELY be looking for a new vet. I would no more allow a vet to speak to me like that than I would cover myself with spiders and dive into a pit filled with scorpions. Ain't gonna happen! So congrats to you for not putting up with that unprofessional behavior.
The first step to finding your best possible veterinary match is to evaluate your own priorities. Take a piece of paper and draw a line down the middle. Label one side "Must" and the other "Must Not". Now, try to put as few things on each side of the paper as possible, because the more "absolutes" you have on that page, the fewer vets will meet your specs. Write down only the deal-breakers - those things you absolutely must have or will not tolerate in a vet. Here are a couple of my "musts" and "must nots", to give you an example:
Must
handle my animals appropriately
listen to me
have a live person answering the phone
Must not
handle my animals roughly
dismiss or ignore me
have an answering machine answering clinic calls during business hours
not return phone calls within a reasonable time frame
And here are a few things that I don't appreciate but will tolerate in an otherwise decent vet:
overcharging
gruff disposition
limited skills, as long as the vet is willing to research unfamiliar veterinary issues
minimally equipped clinic
There are many more veterinary characteristics to consider, of course, but you get the idea. You just need to decide your own deal-breakers so that you can easily eliminate the vets who don't make your grade.
Once you've got the candidates whittled down, then start talking to other pet owners to see what they have to say about the vets left on your list. A vet's reputation will quickly become apparent when you start chatting folks up. Use this step to identify two or three vets at the top of your list.
Now it's time to visit the clinics and meet the vets themselves. Call each clinic and tell them you're in the market for a new vet. Ask if you can come in for a tour of the clinic and to meet the staff. Hopefully, they'll also offer to let you meet with the vet, though some vets may not "donate" their time in that way. If not, then you'll need to make an actual appointment in order to meet the vet. Take Bella in with you for a routine check so that you can see how the vet and staff handle her. During the appointment, take the opportunity to ask your vet as many pertinent questions as possible such as:
Do you offer after-hours emergency service?
Do you provide referrals to veterinary specialists, if necessary?
Will you provide prescriptions, if requested?
If I find veterinary information on the Internet that's specifically relevant to Bella's health, would you be willing to review it?
etc.
Think up your own set of questions depending on your own priorities and needs. Do this with each of your top veterinarians. Seeing how each vet handles Bella, relates to you, and answers your questions should help nail down your best veterinary choice.
Good luck!
Laurie