Please help me in customizing registeration process

Hi WP people. I am developing a membership website in wordpress.

Problem No.1: Have to create a Sign Up form with email id only (i.e. without using username as input)

Problem No2.: Have to create Username with a fixed prefix and a followed by a random number (preferably ID number of the record table wp_users)

Please suggest me any ideas. (either php code or plugins)

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