It depends on the airline, but for mine we have to report for duty an hour before departure.
I don't struggle with swelling a lot but I know some people wear compression leggings or socks. On flights we are standing for hours working so it's not quite the same as a passenger who is sitting for hours on end without moving.
Yes, we work with different people all the time. Often when you are assigned a trip, it is the same people. So if I am scheduled to work a flight to New York City, and then New York City to Montreal, then stay overnight, and then Montreal back to my home base, it will likely be the same people for all those flights. Then the next time I work, it will be a totally different set of people. So you have to be comfortable working with and interacting with new people all the time. Of course after a while you start to see the same faces and get to know people.
The biggest thing when applying is to emphasize your customer service, communication and problem solving abilities. If you get invited to an interview, you don't need to be bubbly or overly cheerful but they are looking for someone who can work well and communicate with a wide range of people. So be warm, friendly and professional.
I encourage you to apply! Best of luck in your application!