Hi ZuZu,
That would be a good idea!
Congratulations on your new mattress 
You certainly made a great quality/value choice and I’m looking forward to your feedback when you’ve received it and have had the chance to sleep on it for a bit.
This would be a personal preference rather than a “better worse” issue. Both can make good choices in the right firmness level for any weight range. There is more about the difference between Dunlop and Talalay in post #7 here.
The only way to know how any combination of layers will feel is based on your own actual experience but of course adding a soft topper to any mattress or combination of layers will make it softer. It would also be a good idea to be cautious about adding too much softness/thickess though because it can be risky for alignment. There is also more about the effect of a separate topper vs having the same layer inside the mattress in posts #3 and #4 here and in this topic.
When you can’t test a mattress in person then one of the most important parts of an online purchase is a more detailed conversation with a knowledgeable and experienced manufacturer so you can discuss these types of questions and they can help “talk you through” the options that would have the best chance of success (see mattress firmness/comfort levels in post #2 here).
Phoenix