Kitchen Floor Tiling / Preparation - Tile Advice

[schakal]

I have a project of tiling my kitchen floor in my hands and i want to do it right at once rather than going back to it in years to come.

The sub base is made up of sand with DPM on top then 50mm cellotex and around 80-100mm "screed"/concrete
Now i am not sure i am using the right word for this. Basically its a premix mix that came on a cement truck and was poured in using wheel barrows by the contractor the same time he was laying my garage slab . Sub base was laid in October 2016 ,i am guessing it should be fairly dry by now.
The surface of the sub base was fairly powdery after it was dried and recently reading up about this on this forum it sounds like LAITANCE.
How do i remove this ?
After removing do i have to prime the surface before tiling ?
if so ,what primer do i use ?
Do i need to use a self leveller even if the surface of the concrete/screed is level ?

Also i have decided on 20cm x 20cm floor tiles.
I want to use the smallest grout gap possible . On a little dry lay ,the tiles look fairly straight.
What is the minimum gap can i get away with ?

thanks for reading

 

[schakal]

here is a comparison of sections of the concrete.
photo 1 , left covered with dpm
photo 2 , left uncovered and walked on where most of the powdery stuff has eroded leaving hard concrete.