Yup, if Sca is a runner (off the course), I'd find something where he can't run off. Koji rarely ran off, but at our first Show n Go (non-competing practice trial), there were no fences and buh-bye, off he went.

So embarrassing. BTW, he did it even when there was a fence around us (he went to visit the judges a few times, ran out the gates, etc).

But you just have to laugh about it.
Since Sca is like Koji, majorly food motivated, my suggestion is this. You don't need to wean them off of food, however, get him to run sequences before you treat him. i.e. put the bag somewhere (not on you), run a sequence (any amount of obstacles), then go to the bag and treat Sca. That's what I did in class. I stopped carrying treats and started treating him at the end. The only time I carry them now, is when I'm working on something specific (like contacts). This basically taught Koji that he has to run the whole thing before he gets treated.
I also used to use treats prior to our run (like still treating until I had to go in the ring), but that was to calm him down and distract him, I think that might have been a mistake, lol. But I'm not sure yet. Because when I run out too early, it sucks, cuz then he gets crazy way too early.
When you get closer to being ready, you should see if there's any show n go type trials in your area. They may call them other things. Less pressure, more fun, a chance to see what Sca will do, and to desensitize him to the whole trial atmosphere.
I know that didn't answer, flat or arena style, but like Ashley said, it all depends on the dog. And well, I've never been in an arena style, so it doesn't matter. The one arena I've been in....the was still a lot of foot traffic on the main floor, so it was like a regular flat trial anyways.
FYI. If you have time, check out some trials with Sca. Don't compete, just spend an hour there walking around, and see how he reacts, it also helps to desensitize him a bit.