Well its obvious you have computer access to the internet. There is a ton of information including the rules in cowboy action shooting. Do a search for SASS or CAS. I'm sure you will find what you want. You can also search for "handloading for cowboy action shooting".
If I recall, load velocities can not exceed 1000 fsp but don't quote me on this. Being that speed is a factor in this sport, you may want to shoot the lightest load you can and still have good accuracy. These targets ain't far away.
I got all the equip for this and never got a chance to go. I had a pair of Ruger Vaqueros (stainless with consecutive s/n's), a Marlin 1894. these were in .44 cal. and a Balkial coach gun 12 ga. I sold the Rugers and the shotgun but I kept the Marlin. I am considering using it as a backup deer rifle (at least that is my excuse). Anyway, its a neat rifle.