The system will directly re-order the imagery based on the mosaic method defined and the MinPS, MaxPS range defined for each raster. It appears in your case that you have different resolution imagery and the lower resolution imagery is fully covering the higher resolution imagery. As a result the MaxPS of the higher resolution imagery is being set to the LoPS of the lower resolution imagery. This represents the general case that most users want, IE higher resolution imagery at the larger scales, but at smaller scales use most appropriate imagery. To change this you need to change the MaxPS values of the higher resolution imagery to a larger values. As a general rule the MaxPS should not be set to more then about 4-5x the HiPS value though. IE you do not want to be sampling the imagery too much as this can really slow things down. (With larger images that have pyramids this is less of an issue and you will notice that that the HiPS value is larger). So one thing you can do is select the required images and then use 'calculate' to set MaxPS = LoPS*4 (or other value of your choosing). You also need to set a suitable Mosaic Method to define which of the overlapping imagery displays on top. This needs to be changed in the default properties of the mosaic dataset (accessible by right clicking on the mosaic dataset in the catalog window). This may not change the properties of the currently loaded version of the mosaic dataset that you have currently loading in ArcMap, so you may need to re-open the mosaic dataset. In most cases you would probably set the method to ByAttribute and define a field and base value to be used. For example you could create a field called Year and set this to by default display the latest imagery on top. (Set base value to 3000 or something in the future.) Users of the Mosaic Dataset or resulting Image Services can redefined the mosaic method and fields etc.
To Guenters note about the units. Yes all the units are now in the units of the SRS used for managing the mosaic dataset. This is a change from 9.2,9.3 where all units were in meters. Using the units related to the projection does make it more standard in how ArcGIS handles scales etc.
The following can be used to Covent between scales and PixelSizes in meters.
Scale = Cell Size x 96/0.0254
Therefore, if the scale is 1:20,000, the cell size is 5.29 meters.
Cell Size = Scale x 0.0254/96
This is covered in the "Cell size ranges in a mosaic dataset" section of the help.
If using Decimal degrees then divide the meters by 111111 to get an approx value in degrees. If using feet divide the meter value by 3.28.
I'm hoping in the future we can provide a better way to view and defined pixels sizes in different units.
