I have read how if you want to put a std stem in a french frame you must file it down to fit, what about the other way around? Is the stem too much smaller to safely tighten? Has anyone done this?

Other way around. Standard quill diameter is 22.2mm, French is 22.0.

A soda can is 0.1mm (adds 0.2 mm to a diameter) and is commonly used for seat post shims. I have never done it for a stem but it can't hurt to try.

I did it on my UO8. Follow Sheldon Brown's article and it's quite easy.
https://www.sheldonbrown.com/harris/french.html

.... as i said im trying to do it the other way around... NOT a std stem thats too big and gets filed down (like in the sheldon brown article) but a French stem into a std 1" ..... ie it will be too SMALL, one person mentioned using a shim and thats what i was thinking of doing... but ive never heard of anyone using a shim into the steerer tube, so i was wondering if anyone had.

The one time i tried to shim something .1 mm it was a seatpost and I had a hell of a time getting the can flat enough around the post to actually slide into the seat tube. I ended up just faking it. So watch out for ruffled edges on the can.

The stem will tighten, but it will tend to rock in the steerer. You'll also have a space between the stem and the headset locknut that invites the intrusion of water. You don't want to add water where you have unlike metals in contact with each other. I say don't do it.

actually, joking aside, the "reverse tooth file" is close to an accepted technique to raise the surface: knurling. I've never tried it on a stem, but have used it (as have many bike mechanics) on fork crowns. It might not be possible to raise the material by a full .2mm, tho that seems like it's within the ballpark, so... worth a shot.

Is there a particular reason you want to do that? Just curious 'cause it seems like you could dump the French stem on Fleabay and pick up a standard one for next to nothing. 'Course I hate it when people make "helpful" suggestions like that to me 'cause there is usually a reason that I'm asking and I already thought of the obvious.

If it has a wedge it would probably work without a hitch. If it has a cone then no. But in either case you could probably swap it with someone who needs a French stem for a standard one and come out ahead. What brand/size stem do you have?

i was looking at some nice bars on ebay, and i figure why not buy some nice used ones...(im building up a frame part by part) but it turns out they are steel, so ill probably end up getting noodle bars

I have used an aluminum can shim with no ill effects. If you put the shim in part way first, then put the stem inside the shim, then you can slide them together the rest of the way in.