Hairline cracks can be completely preventable and depend on a lot of factors (concrete mix, structural slab and ground quality) but there are few best practices that can be undertaken to avoid them. You should consult with your engineer and concreter about this in detail.
Here are some articles with some information on this: http://www.cscscreeding.co.uk/2012/01/controlling-shrinkage-cracking-and-curling-of-screeds/
For screed, you can either create depressions in the structural slab for the screed layer clear of fixtures such as walls, island bench, cupboards, etc, where you want the heating/cooling to be located, or alternately, you can run a screed layer over the whole of the structural slab but you will need to insulate the slab edges otherwise you will leave a conduit for heat loss/gain in winter/summer.
The insulation layer is 25mm and the circuits are 16mm diameter, hence 41mm together. Therefore, you should allow at least 50mm for the screed above the circuits, so a depth of at least 90mm is required and you can apply more screed up to 60mm with total height of insulation + circuits + screed up to 100mm. The pros/cons of less/more screed are:
- Less screed will be more responsive to initial heating/cooling
- More screed has more thermal mass to radiate/absorb heating/cooling into the evening
If you go with thinner screed, then it should be reinforced. You can also use additives or polypropylene reinforcing fiber.
Here is a brochure on the insulation: http://www.dctech.com.au/uploads/54468/ufiles/Product_page_PDFs/DCT_GA_300_Under_Floor_2_Page_Brochure.pdf