The SF G3's do indeed have slightly longer tailcaps (or perhaps just longer grooves actually, giving the perception of being longer to the eye, not 100% sure as I don't have one close by to compare). I cannot see any reason for this other than aesthetics, being that the G3 is obviously a longer light overall??
In my experience, they are in fact interchangeable which I assume is what you are getting at here (the G3 cap looks just a tad out of place on a G2 IMO).
I have NO experience with installing a McClicky in one, though I have converted many G2 tailcaps into reverse clickies using a switch from DX; remove plastic retaining ring & old switch guts, install switch boot flavor of your desire and thread in new switch. Simple. I have needed to trim down the nipple inside of the new non-OEM switch boot so it does not push down on the button upon tightening the replacement switch into place, this is nice because you can essentially adjust the sensitivity (amount of force required and amount of throw) before the switch will activate. It is trial and error, (trimming off tiny amounts and testing each time) until you reach your desired feel. I cut it completely off if I pocket the light a lot to avoid accidental activation if I forget to lock it out.
From what I have read the McClicky installation is similar, I think you need a retaining ring or something (don't quote me on that), maybe there is a kit available for the installation?? Alternatively, you can buy a G2 tailcap with the McClicky pre-installed and sell your extra cap in the MP or keep it should you have the need for the tactical momentary with lock-out as originally designed by SF.
Hope this helps some!!!