Hi Odile, I only use the one ball head and it's the Triton PH22
http://www.teleskop-express.de/shop/...belastbar.html
Sorry for the overseas link but it's where I got mine from.
It has a max load of 3kg which is plenty when used between the unit and your camera.
As my tripod has a 3d pan head there isn't much call for the second ball head between the tripod and the Polarie but if you do need one then I'd just up the rating to any ball head that could take around 5kg.
I do have a manfrotto mini ball head that will work too but I feel it's just too small, even though it'll probably handle the load.
If you already have a good tripod then there is no need for the kit tripod which I think is overpriced anyway. I'm sure it's very nice but also very expensive for what it is.
So you can use any that you find and really just as long as the weight rating is enough.