If this was applied to card with numbers or letters on it would it make it difficult to photograph?

A diffraction grating on a photographic camera lens makes rainbows which are more noticeable on the sides of intense light. Numbers or letters are less affected, because less intense in brightness.

Is it possible to transfer onto silicon?

Yes, but not directly. It seems to stick

Is this shipped on a roll? Or is it shipped flat, with a backing? Or is it just stuffed in an envelope?

This was NOT "shipped flat, with cardboard backing, in a padded envelope" as was suggested by the seller, but (as other reviewers have answered) by itself in a flimsy celliphane bag causing multiple scratches and folds. Buyer Beware!

Can I cut this down into smaller sizes? Looking to build spectroscopes with elementary kids.

Yes, this can be cut down. We too made a spectroscope with it. Cut just a bit wider than the opening; not too much wider as it doesn't wrap well around the sides of the tube.