Hi momabena,
You are certainly heading in a great direction and as you know I think very highly of Shawn and SleepEz.
You won’t “need” the mats on top of the slats no because the gaps between the slats are less than 3" and the mattress will fit and be well supported on the bed without the mats. Having said that … the mattress may be a tight fit and with the slats it may be easier to make the bed and tuck in the sheets (see the comments here).
The “best” way to decide on the layers that have the best chance of working well for you will be with a more detailed phone conversation with the manufacturer themselves (in this case SleepEz). They know more about their mattresses and the options they have available than anyone else (including me) and are in the best position to give you the guidance that you will need to make the best possible choice (see mattress firmness/comfort levels in post #2 here).
They are great to talk to and you will find they are very patient, “no pressure”, and knowledgeable with any questions you may have. 
Phoenix