Definitely don't mix the klonopin for your first time, that would be dangerous especially since you don't know where your tolerance for the oxy is. As for your second question, there's a lot of people who think its stupid to snort pills because of binders and such, i for one enjoy the immediate rush. You should also take into consideration that the BA of oxy is higher when taken oral, so you'll be getting more out of it and probably a longer high. As for 20mg's, i think thats an ok starting dose. To be safe maybe you should start with 10, you can always work your way up and it won't ruin anything.
Have fun and be safe.