Given that you control the weres while they're in dwarf form you can order them to dig out a remote corner and then brick themselves in, leaving the rest of the embark to the fortress. You can even arrange for the weres to atom smash themselves.
It should be noted, though, that fortresses that have taken heavy casualties make migrants wary of moving to such a dangerous place, so it may take a while before anyone is prepared to brave the danger. For that reason it might be better to keep the weres bricked in until you're restocked the fortress with dorfs.
It should be possible to run a fortress with a single dorf, but everything takes a long time to do, and you'd probably have to micro manage it by enabling and disabling jobs, or the bugger would spend the time doing something unimportant (like manually hauling rocks to a stockpile rather than the things that actually need to be done in a timely manner). Stress becomes an issue after a while, as pining for friends (which probably have never existed unless one of the starting 7) or family (which the buggers are are completely incompetent at acquiring) may send the dorf into madness (unless mooded, as mooding protects against insanity, so there'd only be tantruming and/or oblivious stumbling from time to time).