You always want to use "Schedule 40" PVC if you use PVC. It is stronger and more resistant than the regular stuff. It's safer since it's less likely to shatter when hit by your Mini.
Another safety feature to keep in mind is that the poles need to be held by cups so they can easily fall if hit. If the rails cannot fall easily, your horse can seriously hurt his legs if he hits them.
The AMHA rulebook states the jumps must be 5 ft in width and heights vary depending the class. Hunter jump 18-24" and jumpers start at 18-28", but can go to 44" in later rounds of the jump-off. AMHR jumps are 12-24" for hunters, but jumpers can go up to 30" in the second round.
There are lots of decorations ideas if you look through the jumps on our site. www.MiniDreamsProductions.com