For FlatGround:
1. I definitely don't limit kids of any age posting. Younger players should be posting mostly for advice from MLB players, coaches, etc. who follow FG..and should expect to get some honest feedback. Part of my goal was for kids to get free advice (note: not all advice is good advice 
). I would put whatever the player thinks puts him in the best light. Put yourself in the viewer's shoes. What do you think a coach wants to know about you as a player? What makes you different? How can they rely on what you say? [That's where measurables come into play] No harm in putting their travel/hs coach's info in the post, but it may take up valuable room to put measurables, which is the most important.
2. You can post whatever video you have. PG, PBR, or in your back yard. Whatever helps put you in the best light.
3. To get noticed, post good clean video and measurables. Ideally, with radar gun info, Rapsodo/trackman hitting, Exit velo, 60 time, grades...any and every thing that you think a coach will care about and believe. The more proof you have in the video, the more credibility it will have.
Here are some good examples and helpful hints of what to do and not do (hope this helps!).