H Drive questions

[Tottanka]

Hey all,
Im looking for a few tips from anyone experienced with designing and using an H Drive.
First of all, are there any problems with driving in a direct straight line with that kind of drive? I'm thinking the Omni's sliding ability will make driving less "expected" than the usual traction wheel tank drive. If so - how do you solve that?

Second, how much torque do I need to use for the non conventional direction of movement? Is the same gear ratio as other wheel acceptable, or would i need more?

Last but not least, do i have to install the 2 non conventional wheels on the center of geometry/mass of the robot in order to slide well, or is would it be OK to install them at say, 1/3 of the length on the robot towards the back side?

This is all assuming i'm using 4 Omnis in a tank drive style (X movement) with 2 more added at the back of the Drivetrain (Y movement).

Thanks!