Hi cg22,
While I’m not Jeff, I hope you won’t mind me commenting here. 
The polyfoam you linked to is only 1.2 lb, a lower quality item. As you’re going through so much work rebuilding this mattress, I’d recommend sourcing a better piece of foam at 1.8 lb density or above.
No, you should have no worry there. Latex has been used next to polyfoam for decades. Latex has a high coefficient of friction and is quite grippy, so placing it directly against the polyfoam layer will help hold it in place. Manufacturers will often use a very thin layer of an approved spray-on adhesive along the perimeter of where the foam is placed to help secure it into position, but you may not be interested in doing that.
Density and ILD tend to correspond with each other in latex, with a higher density resulting in a higher ILD. The ILDS of Talalay are not “directly” relatable to the ILDs of Dunlop, as Dunlop will have a higher compression modulus at the same ILD of Talalay. For the first 25% or so of compression the Dunlop will tend to feel a bit more plush than the same ILD piece of Talalay, but as it is compressed further it will “firm up faster” than Talalay. There is more detail about this in post #6 here. So if you were comparing two pieces of 24 ILD latex, the Dunlop will “feel firmer” than the Talalay piece.
If your piece of Talalay is from Radium (Vita Talalay), N4 would indeed be quite plush, approximately a 13 ILD. If the piece of Talalay latex was from Talalay Global, it would be firmer, in the range of a 30 ILD. For Dunlop latex, using Arpico’s Dunlop as a guide, a density of 75 kg/M3 would be an approximate 24-28 ILD rating.
As for which you should choose, that is of course up to your own personal comfort preference, and something I unfortunately can’t predict for you. I would check into any potential return/exchange policies that any foam retailer might have, just in case whatever piece you purchase doesn’t turn out as well as you had hoped it might, as you are replacing foam that is not only old, but also a different type, in your mattress.
I hope that helps give you a little more guidance.
Phoenix