PEX Tubing questions

All the "zones" should be close to equal in length and should not exceed 250' as previously mentioned. This will keep the return water close to the same temp of each zone and not shock the boiler.
Do not install the lines at the bottom of the slab with plastic staples on top of the styrofoam below the slab.
Try to keep the lines on top of the rebar with all return corners well tied. This will minimize damage while you are pouring the cement.
Pressurize the system at 20-30 psi through a common header connecting all lines before the pour.
Have a small propane torch handy or extra coupling should a line get damaged during the pour. The concrete will bubble should damage occur.
I highly suggest installing a slab sensor. I installed two, this way When and of if one doesn't work you will a backup.
Are you using a electric or gas boiler?
Cheers.