Now this is a subject i use to struggle with as to what to do, so this is what we do,
Basically, join the buggers up wherever possible. Shorts are kept as shorts on there own spools with a few exceptions, I Have two Disney 600ft spools made up of many Disney Trailers & extra long promo's, these include, Poccahontas promo, Lion King Promo and trailers, 101 dalmatians,(live action version), a great Virgin cinema opening, Dinosaur trailer and promo, Who Framed roger rabbit, An American tail, Fantasia 2000, Finding Nemo, Treasure planet and Jungle book 2, Hunchback of Notre dam, Lion King Special preview, Little Mermaid sing a long, Tarzan special preview, Toy story,
These trailers fit onto 2 600 foot spools and make great half hour entertainment in there own right. The reason why these are quite special is simply when my Children were growing up these were what i collected over there growing years, all films that they grew up with.
We also like to splice a trailer onto the start of our feature films and also make up a couple of short shows with,(for example), The making of the Godfather promo is joined with a 400ft cut down with a trailer at the start.
Most feature films are joined onto 800 or 1200 spools with the original boxes carefully stored in case we decide to sell any in the future.