first can you use a standard jump bike frame?
Yes, you could do trials on any bike with good brakes, its just not ideal and would be harder to learn.
and i have noticed that if they have a v-brake it is on the back, can you put one on front and back
Yes you can, a lot of people do, a decent set work well and are light.
also can i use 26" wheels, and if i do will i be able to do the kind of things the 'pros' do after a lot of practice
Yes you can, after lots and lots of practice.
last question: i am using rigid forks but i have 2 types in my arsenal a bit like these two:
http://cgi.ebay.co.uk/eXotic-JUMP-CREATURE-Carbon-Dirt-Street-Fork-40-5cm-/200506533667?pt=UK_sportsleisure_cycling_bikeparts_SR&hash=item2eaf1ee723
and
http://cgi.ebay.co.uk/GT-Rigid-Fork-Disc-mounts-1-1-8th-steerer-/170531847316?pt=UK_sportsleisure_cycling_bikeparts_SR&hash=item27b47d7c94
which style should i use?
I dont know which ones you should use, but again the geos are probably not ideal for trials.