The frame rate depends on USB controllers present in a PC. Looking at specs it seems that your laptop likely has an additional USB v3 controller. So there's a good chance that 6 cameras can stream at 60 fps provided you connect them as 2x per each USB port.
The actor model in Mocap Studio resembles a normal fitting clothing, so there is no advantage in wearing tight clothes. The main thing is that clothing colors are solid.
You can position RGB cameras any way as soon as they have a good intersection of their fields of view. Actually having a camera looking down can be beneficial for certain motions like crawling.
The main requirements for background are
- A reasonable contrast with actor clothes
- No moving objects
If you can get this without backdrops then you're fine.
Action cameras can provide better accuracy of tracking fast motions due to higher FPS. Otherwise, the choice between two options comes down to convenience/cost or what you've already got in your hands. Action cams workflow is easier in set up and recording at the expense of more complex processing. You have to merge and sync individual recordings as a separate step, and usually remove distortions in wide FoV videos.