Unfortunately this is a limitation with these two modes. They require the segment sizes to to be the same to correctly scale the segments. One option though is to use an expression to scale the final segment in the section to make sure it fits. In that way you can randomise the others. This requires two techniques: One to detect the last segment, and another to scale it.
If you want to recreate count mode then detecting the final segment is easy. You can simply test the current segment count value. To do this you would use the variable SegmentXCounter. To scale the final segment you need to find out how much space is left along the spline. To do this use XSectionLength and XSectionPosition. The first of these returns the length in scene units of the current spline section, the second returns a scalar (0-1) of the splines current position. Using these you can work out exactly how much space remains and use it to scale the final segment.
I've attached an example of this for you to take a look at, this can be adapted for more complex uses.
I hope that helps, please let me know if you have any further questions.