Just two days ago I replaced the 12 inch wheels on my 1994 metro with stock 13 inch metro wheels from a local junk yard, and I used the stock nuts that were on the metro with no problems. (Yes I had a hard time finding them, and no I don't know where there are any more 13 inch metro rims.)
CAUTION: This may not be safe in all cases so check it out carefully!
Because the cone shaped depressions in the new wheels are larger in diameter than in the old 12 inch wheel, the old smaller nuts will nest deeper into the new wheels, and will have less bearing surface between the cone of the nut and the cone of the wheel than would the larger special nuts available.
I checked and verified that the bearing surface between the nut and wheel was great enough, and that the nuts would not bottom out before contacting the wheel.
Without the wheel on, put the nuts on the studs and run them down all the way. Remember that the nuts can bottom against the threads or against the end of the nut, if it is a blind nut, as mine are.
Mine went all the down, with the end of the cone on the nut touching the drum, without binding or bottoming.
Place a nut in a cone of the wheel, lay a straight edge across the flats of the wheel, and verify that you have a gap between the tip of the cone end of the nut and the straight edge. I had about 1/16 of an inch on mine.
If in doubt, buy a set of the special nuts.
Team swift has a load of information on this and on on other wheels that will fit a metro, etc. at
http://www.teamswift.net/viewforum.php?f=10
According to my 1994 factory service manual the wheel nuts should be torqued to 60-Nm (44 lb.ft). I always hand tighten them, and then re check them again a couple of times after driving a few miles. I won't let the guys at a tire shop get close to the metro with an air wrench. It already has some replacement wheel studs on the rear from previous owners, and I don't want to replace any more.