The tricks you can do really depends on how it's built and the springs. A lot of my friends have those shitty walmart trampolines. It's hard to get enough air to stall out a flip. I have a round one as well and you can go huge on it, dubs aren't that hard.
Would love a real nice rect in a pit though. That would be awesome.
From what the guy who sold my friend one said, is that rectangular ones (outside in the ground) don't do as well. Something about the lack of airflow causes them to not be as springy as if they were above ground, or even not completely submerged.