You should choose whoever you want to be a close part of your special day. There is no need for any of them to be single. The only exception is your Maid of Honor – if she is married then her title will be Matron of Honor.⠀⠀⠀⠀⠀⠀⠀
It’s perfectly fine to select single or married people. The bridal party usually pays for their own attire but if you select a very expensive bridesmaid dress you can pay for some of the expenses as a generous gesture.