There's a section in the code that specifies the height of the stand pipe riser for a washing machine. I don't remember, but if you search a little here, it has been mentioned many times. You don't really want the trap below the floor. Any threaded tapered connections you make need either pipe dope or teflon tape to seal them. You could use a no-hub connection to make the copper to pvc junction, or install a female theaded socket and then put a male threaded fitting on the pvc. The no-hub is faster and easier (but might cost a little more).