I don't use resistors on my MMC. I find these waste energy and cut down on my spark length. I use CDE 942C series 1.6Kvdc 680nF capacitors, I use 20 per string 34nF @ 32Kvdc x2 . I run 12Kvac @ 240mA. I have been running my MMC now for 2 years and have had 0 problems.
Quite often many people are scared about residual charge in the cap bank this is why people add the resistors. It bleeds charge away after the unit is turned off. You can do this with a discharge wand MUCH cheaper.
Like I said I have never used them. It is a waste of energy.
If you feel like you want to protect your cap bank, I'd might make an extra gap that one could adjust.
maybe say to be safe a 3.5cm gap 35Kv. I would also make sure the MMC total DC rating is around 3x that of the input voltage. 12Kvac -- 32-36Kvdc. Another factor I would also check is the tolerance rating of the capacitors most are 5%... which is ok... CDEs on the other hand are 10% and can handle much more voltage spikes.