Hi jege41,
That would really depend on a lot of factors including the layering in your mattress, the thickness and firmness of the current comfort layers, how your body type and sleeping style interacts with your mattress, and perhaps most of all what you consider and perceive as being “medium to soft” because each person will have a different perception and definition of this.
The best way I know to choose which topper may be best for your application are the guidelines in post #8 here and post #4 here.
Generally the choice of material would be between fast response latex (or perhaps polyfoam for a lower cost, lower performance, and less durable alternative) and memory foam (either “regular” memory foam or gel memory foam) and would be a mattter of personal preference based on which you were most familiar with and preferred.
If you are asking for my personal preference it would be talalay latex but this is just what I personally prefer and doesn’t mean that the next person would have the same preference and on an individual basis it’s certainly not “better” than other options that someone else may prefer.
The best way to maintain good alignment is to use “just enough” in terms of thickness and softness to relieve pressure in all your sleeping positions (particularly the more pressure prone positions such as side sleeping) but no more.
No, I’ve never worked in either the bedding or foam industry. I also wouldn’t have any time to work at anything else beside this site … it’s a 16 hour a day 7 day a week project and has been for over 2 years now. The information here is not the result of my own work experience but the result of thousands of hours of conversations with many manufacturers and “experts” around the country, thousands more hours of research into all the topics that are discussed here including research into various foams and other materials, and hundreds of hours of testing mattresses to help 'translate" the information into more “real world” terms.
One day I may get back to a more “normal” life … but in the meantime I do love what I’m doing 
Phoenix