All my dropins q5(defunct),r2,r5 are all mounted on 16mm stars. The only 20mm star was in my ex romisen RC-C8 twisty zoomie. A 16mm star is indeed better to aid aligning it properly. I use the goop fujik from dx. Not awesome but does the job. Be carefull as to not make it float in it. A small amount and hard pressed on the base is the best metod i believe since heatsinking leds can't be that different from CPU's, all in all it is the same principle, unfortunately there is nothing to keep pressure on the star.
The reflector screws down and bears on it usually in a P60. At least most of mine do, but they are really bearing on the solder where the leads connect to the star.
Yap, thats correct. A nice rubber washer for plumbig but thin could be just the thing to put enough pressure and put just a smear of thermal glue underneath. That along with alu foil is actually all you can do to aid thermal transfer to p60 hosts.