25x70mm battens for suspended ply subfloor for tiling? which wood?

[Mirjam]

We want to make a suspended plywood floor in our ground floor for tiling:
- existing concrete
- vapour and radon barrier
- 25 (H) x70mm (W) battens and insulation boards
- 18mm hardwood plywood
- 6mm backerboard
- 30x60x9mm porcelain tiles

DO YOU THINK 25mm BATTENS ARE ENOUGH? (I read that the minimum is 40mm (H), not 25 mm. The screws that hold the wood down would still go 2cm into the battens, so could there be any problem?

WHAT KIND OF WOOD should we use? maybe it is better to use some more compact wood than pine? since they are not so high??? any suggestions? I read that hardwood is expanding and contracting more, so maybe that would be counter productive?

IS IT GOOD TO USE HARDWOOD PLYWOOD? or should we go for a normal plywood board?

Thanks,

Alexander and Mirjam.