1. 10th planet is a no-gi Bjj style founded by Eddie Bravo. It's supposedly designed around MMA, and uses some flashy vocabulary to describe its techniques. Like Electric Chair, mission control, and Ghost pass. Eddie Bravo is famous for tapping out Royler Gracie over a decade ago as a purple belt in Brazil.
With CSW I take it you're talking about Combat Submission Wrestling. That style was founded by Erik Paulsen, and it's a combination of several grappling styles along with striking styles. Erik Paulsen is considered one of the best grapplers of his generation, and one of the men responsible for bringing Catch Wrestling back to the forefront along with Josh Barnett.
10th planet is going to be entirely grappling, while CSW is going to be Grappling and striking. Both are excellent MA organizations. You can't go wrong with either one provided that their good schools.
2. Yes, 10th planet is really Bjj. Eddie trained under the Machados, and is as legit as they come, and he's produced some pretty impressive students. Eddie's style of Bjj is a bit different than standard Bjj because it's completely no gi, and Eddie has added his own variations to several techniques. His school also has a very unique culture that you won't find in other Bjj associations.
3. If you're new to grappling and MMA, I wouldn't recommend it. You'll simply confuse yourself and hinder you're development. You should definitely take one or the other and dedicate as much time as you can to learn the system.
4. I know it's cliche to say, but that's really up to you. Go to both and see which place you fit in better and dedicate your time there.
