[FTC]: Questions about 4 motor drivetrain

[Techhexium]

Hello, I am currently prototyping a basic direct drive 4 motor drivetrain with 4 traction wheels (4 inches). Here are my questions:

1) Is there a difference of speed for using 4 motors instead of 2? Like if the robot is heavy, would the 2 motor drivetrain be slower than normal but the 4 motor drivetrain not be as affected?

2) Is there a good way to put in two wheels between the 4 wheels for a six wheel configuration? They won't have any motors installed on them. I have read from the FTC Robotics book that these two center wheels should be slightly lower than the outer wheels (these wheels have motors). I have done that but then b/c the outer wheels aren't really touching the ground they just move in place.

Pictures:
http://imageshack.us/photo/my-images/815/img5649.jpg/
http://imageshack.us/photo/my-images/51/img5650x.jpg/


Thanks for the help!