If they are in god condition now I would not increase their rations until the last 8-10weeks before kidding and then do it gradually with as wide a range of feeds you can get your hands on - if they do get picky you have a higher chance of having something they still like... You may find that you will need to increase the frequency of feeds (as in three to four smaller meals a day rather than two big ones) to allow for growing kids/reducing rumen space.
Until then maintenance ration, plus carrots/cabbages/apples/bananas etc as you are doing just now. Loads of hay - my goatlings are on just a handful of soaked shreds with a dusting of oats, plus carrots.
I have only got a Red Rockie, and that has been fine for the last 9 years or so but I do give them periodically a mineral/vitamin drench (a sheep one with added copper), maybe twice a year. They are due one about now.
My goats will not touch Caprivite... but every so often I will add some garlic granules to their feed.
Try not to stress them in early pregnancy in particular, and if they do need worming or so I would do it before mating. Watch for lice after mating with a male from a different holding.
Get your kidding stuff ready, and have Propylene glycol - something like Twin lamb drench (I prefer Cetophyton, available from the vet) and a good drenching syringe 50ml like this one -
http://www.fearing.co.uk/cattle/cattle-health/syringes/re-useable-syringe ready.