> 20 Minimum Hfe on a BJT Power Transistor

To calculate power dissipation in a bipolar transistor you just need to know the collector-to-emitter voltage when it's passing the current that you want to control. This may be called the saturation \$V_{CE}\$ in the data sheet, and it will typically be less than 1 V, perhaps as low as 0.3 V (assuming that your providing enough base current for the transistor to be conducting really well). Multiply that voltage by the current being switched and you have your power dissipation.

Just FYI, there is no "channel" in a BJT. Bipolar transistors work by a completely different mechanism than field-effect transistors. Also, the notion of a fixed source-to-drain resistance for an FET is a highly simplified model and is really just an approximation of the true behavior.

The emitter follower, because of the negative feedback is pretty linear amplifier and is used for large signals all the time. Of course, if you want it to be even more linear, some special schematics have to be used but it actually depends on the application.

Also, the emitter follower has voltage gain less than 1 but the current gain is pretty high and as a result it is amplifier.

In your question you didn't specify what exactly the signal and load is, but on 10Vp-p and 50ohms of load, you probably will need some push-pull emitter follower as a power stage.