I like to set my bikes up as versatile units, I usually operate them with a melta and plasma (or grav-gun) and a Assault Bike with a Multi-Melta. I like to max the unit size and then combat squad the units...often putting the meltas together for anti-armor and then use the other squad with the light armor weapon separately.
Another good way to use them is in minimum size units (3 man) and spam like weapon sets for different missions.
As for "how" to use them. That really depends on the rest of your army. I run a pretty mobile "mechanized" force, with Razorbacks, Rhinos, and Bikes. The bikes tend to be my first strike units...either charging forward to destroy key threats (heavy armor pieces, etc), to harass and pick apart the enemy's least mobile units, and to shape the battlefield. With the Mission Objective cards they also work well to capture objective points since they can really motor across the battlefield.
I know other people use them differently, especially if they are deployed in a all bike army.
As for Khorso...he makes White Scar bikers very good with the scouting move, even more value if you plan on doing Razorback or Rhino spam as well.
If you want to make a Khorso model you will have to do a conversion.
Here is the one I use (as part of my custom chapter...and I still need to get him painted /sigh):
here are a few other good conversions: