If you mix tracks from different albums, they will be volume leveled appropriately. I do this all the time and it works quite well.
Tracks from the same album are leveled to a common target so that you can listen to a whole album and hear the differences in volume level from track to track as the artists intended the album to be heard. I think JRiver did this so you wouldn't have to turn volume leveling off every time you wanted to listen to an album as a whole.
When I learned about this feature, I thought that the "album leveling" only applied to sequential tracks from the same album. However, it appears that this album leveling is done to any tracks from the same album that are in the same playlist, next to each other. I'm really not 100% sure. Maybe someone else can comment further.
I just know that volume leveling works well with a mixed playlist. You might also want to turn on Adaptive Volume (peak level normalize). That will bring back some of the lost gain from volume leveling.
Brian.