Adding to Raul's usual wise advice, don't be too shy about going heavier than even Moerch's heaviest (14 grams). I've added the supplementary brass cartridge carrier to my 18 gram eff. mass Schröder - bringing it up to 23 grams. I think this was a touch heavy, but the standard 18 grams (using the aluminum carrier) was certainly not.
The numbers would tell you to not expect this. We know better than to stop at the numbers (grin).
For your reference, a nickel weighs about 5.6 grams (tack onto the top of your headshell with a small piece of Blu-tack). We did this to a 9" Moerch red dot arm - to bring the mass "up" to 11.5 grams. I'd consider this the minimum for the silver based ZYX Airy and Universe cartridges (no experience with the other ZYX cartridges).
Thom @ Galibier