Can I change from short to long derailleur to increase cog size I am have a tiagra 11-25 10 speed cassette with short cage. I am a poor climber and looking for extra gear; I would like to go 11-32 for this year until my climbing improves, will the short cage derailleur take a 32 teeth cog?

If not what 10 speed derailleur is recommended?
It doesn't look like you'd be able to use an 11-32T cassette at all with a 10-speed Tiagra rear derailleur.  From [Shimano's documentation] (http://bike.shimano.com/media/techdocs/content/cycle/SI/SI_5XN0A/SI-5XN0A-001-Eng_v1_m56577569830746860.pdf), the largest sprocket that either the short cage or long cage version supports is 30T.  If you wanted to use their 12-30T tiagra cassette, you would need the long cage because the tooth difference has to be 16 teeth or less for the short cage, whereas the long cage supports a tooth difference up to 22 teeth.

If you really want to go up to 32T and stick with a road derailleur, it looks like you'll have to upgrade to the newer 11 speed stuff and get the long cage version.  The [11 speed Ultegra] (http://bike.shimano.com/content/sac-bike/en/home/road/shifting---braking/rear-derailleurs/rd-6800-gs.html) derailleur looks like the only road derailleur from Shimano that would support an 11-32T cassette.  Incidentally, that's the derailleur that came stock on my 'cross bike and it's got an 11-32T cassette on there.