If you've employed a builder to do the work then stick with him and his team, getting your own plumber in to do bits instead of using the builders "team" rarely works.
The bottom fitting pushes into a standard 110mm fitting, you wouldnt use the drain connector just a standard socket will do, your bottom photo is a glued one, but a push fit one will do.
The bottom fitting is to connect the supplied elbow to a standard 110mm soil fitting, if your existing soil pipe is going to line up with where the bracket sits on the frame you can just use a normal long pan connector.