What is considered a short runway for the -8?

F25 is standard on the -400 for the Big Airline as it reduces loads (and therefore wear/maintenance cost) on the flap assembly, as well as saving a bit of fuel and reducing noise slightly (lower thrust/slightly less aerodynamic noise).

Flap 30 used for short/limiting conditions (I would say <2500m starts feeling 'short' in the 744, but you can (and indeed should) always check the normal configuration landing distance tables in the QRH to confirm your landing performance for each configuration and certainly should do so if there is any doubt) or low visibility operations (the pitch on final approach will be about 1-2 degrees lower with F30 giving a better visual segment).

All the above applicable to the -400 as I say: I don't know how different the -8 is.