One additional thing to consider when building a drivetrain is a conversion of weight of the robot into pushing force through the friction.
If all wheels are powered, then the full weight of the robot gets converted into pulling or pushing force through the friction coefficient specific to the wheel type and surface that it is driving on.
However, if free spinning wheels support portion of the weight, then it reduces the maximum force that is available to accelerate, push, or resist pushing of the robot.
(See this topic for more details)
As far as the placing of the wheels is concerned: if the robot’s center of mass coincides with the geometric center between traction wheels, then driving is going to be smooth and predictable.
It doesn’t matter if omnis are not the same distance from the center - you will be fine as long as all powered wheels on each drive side are aligned on the same line and have the same surface speed.
It is very easy to prove with a simple geometry of the velocity vectors, but I cannot find an illustration at the moment. Please, beat me to posting one.