Nadia: any stencil will work for a mirrored wall. Just use blue painter's tape, a sponge with VERY little paint (almost dry!) on it -- and have fun!
Nadia....I would also suggest a round sponge brush and dab only.
Painting on glass requires oil based paints, in my experience.
Idea: perhaps you want to make your own stencil-looking stickers by printing on “sticky back”, which is translucent plastic film with an adhesive back.
The back is covered with a thin film that is peeled off after you print on it whatever you want in whatever colors.
Where can I find translucent plastic film?