>How bad are the customers? Do bad things happen daily, or is it rarer than that?
It really fluctuates with the seasons. If you were to get a job in retail right now, I'd say this is one of the least stressful times of year. Assuming you are in the USA (which is where I live), people are more stingy with their money in the period after Christmas and before they get their tax returns. What that translates to, overall, is fewer people, and thus fewer bitchy people. February is probably the slowest month. June and July are really busy, and obviously so is October through Christmas. Typically, the busier it is, the more grumpy and rude people you will find, and the more people will try to rip off the store you're working in/steal stuff. As for how bad they are... I've had a fair amount of bad experiences that shook me up. I think the worst/most memorable was the first Christmas season I worked, I was closing as the only person on register and a lady's card got denied. She yelled at me, telling me I was lying that her card was denied because she spent over $500 on it earlier that day (which is probably why she got denied...), telling me to go fuck myself and she tossed a shirt at me and demanded to see the manager, who ended up telling her the same thing. She stormed off and was cursing at us when she left and left my register a huge mess with merchandise everywhere. It was years ago so exact words are escaping me but I remember I was struggling to hold back tears. There was another time where two stereotypical dudebro douchebags were asking me questions about what I was doing with my life and if I had gone to college, and also asking me where a lot of things were, without showing any interest in buying stuff (so both bothering me and wasting my time). They got kicked out of the store because they had done that to other employees too that day, apparently.
This is not typical. I want to stress that. But I also think you should know that it can get that bad. Usually the worst it gets is people fighting about fake coupons/wrong sale prices/etc and having them swear a few times. Again, depending on the time of year, if you are working customer service or register, expect that to happen either a few times a week or a few times a day (the latter only during Christmas season). The extreme examples I mentioned above will only happen to you a few times a year, if that.
If you can, try not to work the registers or at customer service if the store has one if you're nervous about interacting with people a lot. If you work on the sales floor, you will inevitably have to work with people, but the interaction is much less because you're mainly stocking, making sure stuff looks clean, and only occasionally pointing out where an item is to a customer. If you have to accept a cashiering job to start, so be it, but when you have worked for a few weeks it's not unreasonable to ask to be trained for working out on the floor. There's also back room and signing jobs where you're pretty much on your own, as well as overnight stuff. I've worked a lot of different jobs at the store I worked at, and I fully realize this isn't a thing a lot of you would probably be interested in, but I've noticed that if your store has a food area (like a McDonalds or Starbucks), even though you have to interact with people, they are generally nicer because they know not to mess with the people making their food.
I should also mention that I live in a fairly rural area, and for a large chain. While the store work in is pretty high-volume, considering, I have no idea what it's like to work in an urban area or a small business, and I am guessing it's very different. I did work briefly in a suburban store and people were slightly ruder but not too much (it might have a bit to do with the state it was in, as well).
>Do certain types of people get treated better than others by the customers?
By customers, not really. Especially if you're just ringing up their groceries and/or clothing, they don't really care, they just want to pay for their stuff and leave for the most part. I hear bad stories from pretty much everybody in the store regardless of what they look like or how they act. I would almost say that guys like us have an advantage in a way because we blend in and are innocuous (or at least I feel that way). They aren't particularly likely to say we did a good job (as in seeking out a manager to say so) as compared to the other two examples you posted, most likely, but as long as we do our job and they leave happy, they won't complain or treat us badly (unless they're assholes, in which case they'd be assholes to anybody).
Coworkers and managers are a bit of a different story because you actually have to work with them. That said, like I said before, a lot of people really don't care about interacting a lot and if coworkers do treat you really badly, start making a log of times and exact things they said or did. Most stores have security cameras not just for the customers, but for the employees too, for better or for worse. Unless the management in your store is complete shit, something will be done about it swiftly.
Wow, sorry for the wall of text. I hope this has been helpful.