100g sugar in 1ltr of liquid gives approximately 36 points, 1000g, as recommended in the kit if put in 21ltrs will give 48g per ltr, 17 points. 700g divided by 21ltrs gives you 33g per litre so 12 points. Difference of 5 points which should be around 0.7% volume reduction over the kit suggested volume at 23ltrs.
Now please take all that with a pinch of salt, I am reasonably certain of my calculations but that is the first time I have tried to work something like that out, normally I am trying to add volume and I did it really quickly, I may have missed a step lol!
As for the yeast, just put it all in, shouldn't make any difference as far as I am aware. I have brewed both my kits at 21ltrs with no issues but the mangrove jacks cider I will bee putting on later will be at the recommended 23ltrs.
Out of curiosity, why brew short of the recommended 23Ltrs if you want a lower % cider?