hello andy,
Most important, when creating custom packs is, you also include all voices that are used by particular style. Let's say Eastern pack has 20 styles and 25 voices.. But if you only like 5 styles (for your custom pack), then obviously not all 25 voices are needed. It would be max 8-10 voices (guessing from my experience) that are needed for those 5 styles. Keep in mind, that you maybe don't need to include all OTS voices for each style, because you can very easy assign some "similar" OTS voice on keyboard -unless, for example, there's some "special" voice in OTS that is just needed to be there, so the whole result sounds "original".
Speaking for me, before moving from original pack to custom pack, I check every style for voices. Sometimes I replace some voices (re-voice) before I include style in custom pack. For example, I replace some accordion voice with similar one that's built in keyboard. I do all that on PC of course, because if dealing with many styles, it takes too long (and is too complicated) on keyboard.
My keyboard only has 400MB user space, so I need to be very rational with custom voices. However I never sacrifice voices that are really needed for style to sound as intended.
Bogdan