Hi Davchun,
The differences between Talalay and Dunlop latex are explained in more detail in this post and this post. In general, in a similar ILD, Dunlop latex will feel a bit firmer and not be quite as buoyant as a similar Talalay.
As for picking out the thickness of a topper, go back to the link I shared in may earlier reply and the posts it links to on how to select a topper. Generally, for a “little to fair” bit of extra plushness, a 2" latex topper is popular. For a “fair to a lot” of extra softness, a 3" latex topper is usually more popular.
Phoenix