First, it is highly unlikely that there are multiple shoppers in a location at the same time. It doesn't provide much value to the client. Usually shops have a minimum time between them, so I have had schedulers call and ask if I can move my shop a few days forward or back because they have another shopper they are trying to accommodate or a reshop that needs to be done.
Most companies do not get involved in some dress code because they are assuming that shoppers will act like professionals and wear something that makes them unremarkable. You can dye your hair blue if you are going to a place where many 'ordinary customers' have dyed their hair some bizarre color, but if you are going to a place where that would make you remarkable, don't do it. If you found sweatpants on someone else 'not ordinary' and perhaps not appropriate when you were there, don't wear sweatpants. But you probably don't want to look like you are headed out to the prom either.
If you have not done restaurant shops previously, I would strongly encourage you to start with more casual restaurants where the bill won't break your budget if your shop gets rejected. Restaurant shops require a lot of timings of services, evaluation of server behaviors and more general observations in addition to what you are required to order, how to order it, how it is served and evaluation of the quantity and quality in addition to the appropriateness of price. It gets easier the more you do, but until you get to know how working with the company is for you and what all is needed, don't put just a whole lot at risk.
As for the foods to be ordered. Some companies are very specific, some very loose. Our most frequent require we each order a beverage and we have the option of non-alcoholic, which makes it easier for me to get all my observations rather than have them tend to blur. We usually have a choice of getting an appetizer or dessert to share and we must order different entrees. Without alcohol we can stay within budget and eat well, which is an advantage. Some shops allow no 'take outs', which is not the same as 'no leftovers' and some apparently say you can't even take home leftovers (though I have never seen that). Usually if you need to order something you don't want you can pick at it and decide to take it home where the dog can have it or somebody can have it for lunch the next day. YOU DO NEED TO TASTE AND EVALUATE THE PRODUCT FRESH AS SERVED. Don't like key lime pie but have to order it? Taste and report, for example, "The key lime pie was a pale green color with a dollop of whipped cream on top. The filling texture was creamy and sweet with noticeable tartness. The crust was a graham cracker crust that was soggy with moisture from the filling." Nothing there says I liked or disliked it. I was objective and I dealt with it. If I waited until I got home the crust would have been noticeably soggy, whether it was when originally served or not. Having tasted it I am free to take it or leave it. If you leave it you can have issues of your server saying, "You didn't like it? Here, let me take it off of your bill." Oops, if it comes off your bill you have no proof that you ordered it. So I tend to take it home and dispose of it in my own garbage can.