RF FETs - how exactly are they different from regular FETs?
There are hundreds of types of RF FETs all with their specific (sometimes niche) application. There are RF FETs for high, medium and low power, switching operation or linear. High, medium or low transconductance. Many frequency ranges.

That a certain RF FET is made for a certain application does not mean it cannot be used in a different application. You will have to look at the datesheet to find out if it suits your needs. But some specialist RF FETs can be very expensive so in general you would only use it for it's intended purpose.

"Are they for final stage amplification?"

Some are, some are not.

"Say you have an opamp that doesn't go for the final desired voltage, so one uses these FETs as Common amplifiers?"

You could but as opamps in general aren't considered "RF" using an RF FET with an opamp would be silly. General purpose FETs are good enough for such a task and probably a lot cheaper as well.

"Is that why they are rated with gain and frequency in the page, rather than Transconductance and rise/fall times?"

Yes, in RF the "analog" parameters are important like gain (which is actually the transconductance under specified load conditions), cutoff frequency, input impedance, biasing requirements, noise, etc.