how do i delete unused items?

In home (lower left pane): Select, right-click > Delete or just DEL (but make sure plan is unlocked if items are part of base plan). In furniture list (upper left pane), imported furniture can be deleted, furniture catalogues must be removed - see this thread.

create an opening fora fireplace that is not square is there a customisable rectangle somewhere?

An invisible object imported as a door window, will cut a wall to the correct shape. It sounds to me that you could place your fireplace object half way into the wall (turn of magnetism with ALT) and then insert an opening (eg Service Hatch) around the opening in the fireplace - or am I missing the point?

ok

PS - you really had me wondering for a sec what a "fora fireplace" was, had not heard of those before [:)]